Huberman Lab - Using Light (Sunlight, Blue Light & Red Light) to Optimize Health

Welcome to the Huberman Lab Podcast,

where we discuss science

and science-based tools for everyday life.

I’m Andrew Huberman,

and I’m a professor of neurobiology and ophthalmology

at Stanford School of Medicine.

Today, we are going to discuss light

and the many powerful uses of light to optimize our health.

We’re going to discuss the use of light

for optimizing skin health, appearance, and longevity,

for wound healing, for optimizing hormone balance,

and for regulating sleep, alertness, mood,

and even for offsetting dementia.

One of the reasons why light has such powerful effects

on so many different aspects of our biology

is that it can be translated

into electrical signals in our brain and body,

into hormone signals in our brain and body,

and indeed into what we call

cascades of biological pathways,

meaning light can actually change the genes

that the cells of your bodies express,

and that is true throughout the lifespan.

Today, I will discuss the mechanisms

by which all of that occurs.

I promise to make it clear for those of you

that don’t have a biology background,

and if you do have a biology background,

I’ll try and provide sufficient depth

so that it’s still of interest to you.

And I promise to give you tools,

very specific protocols that are extracted

from the peer review literature

that will allow you to use different so-called wavelengths,

which most of us think of as colors, of light

in order to modulate your health

in the ways that are most important to you.

For those of you that are thinking

that the use of light to modulate health

falls under the category of woo science,

pseudoscience, or biohacking,

well, nothing could be further from the truth.

In fact, in 1903, the Nobel Prize was given

to Niels Finsen, he was Icelandic,

he lived in Denmark, for the use of phototherapy

for the treatment of lupus.

So there’s more than a hundred years of quality science

emphasizing the use of light,

and as you’ll soon see,

the use of particular wavelengths or colors of light

in order to modulate the activity of cells

in the brain and body.

So while it is the case that many places

and companies are selling therapies and products

related to the use of flashing lights and colored lights,

promising specific outcomes from everything

from stem cell renewal to improvement of brain function,

and some of those don’t have any basis in science,

there are phototherapies that do have

a strong foundation in quality science,

and those are the studies and the protocols

that we are going to discuss today.

But I thought that people might appreciate

knowing that over a hundred years ago,

people were thinking about the use of light

for the treatment of various diseases

and for improving health,

and indeed, many of those therapies are used today

in high quality hospitals and research institutions,

and of course, clinics and homes around the world.

One of the more exciting examples of phototherapy

in the last few years is the beautiful work

of Dr. Glenn Jeffrey at University College London.

The Jeffrey Lab is known for doing pioneering

and very rigorous research

in the realm of visual neuroscience,

and in the last decade or so,

they’ve turned their attention to exploring the role

of red light therapy for offsetting age-related vision loss.

What they discovered is that just brief exposures

to red light early in the day

can offset much of the vision loss

that occurs in people 40 years or older.

And what’s remarkable about these studies

is that the entire duration of the therapy

is just one to three minutes done

just a few times per week.

What’s even more exciting

is that they understand the mechanism

by which this occurred.

The cells in the back of the eye

that convert light information into electrical signals

that the rest of the brain can understand

and create visual images from,

well, those cells are extremely metabolically active.

They need a lot of ATP or energy.

And as we age, those cells get less efficient

at creating that ATP and energy.

Exposure to red light early in the day,

and it does have to be early in the day,

allowed those cells to replenish the mechanisms

by which they create ATP.

I’ll talk about these experiments in more detail

later in the episode and the protocols

so that you can apply those protocols should you choose.

But I use this as an example of our growing understanding

of not just that phototherapies work, but how they work.

And it is through the linking of protocols and mechanism

that we, meaning all of us,

can start to apply phototherapies

in a rational, safe, and powerful way.

Before we begin, I’d like to emphasize

that this podcast is separate

from my teaching and research roles at Stanford.

It is, however, part of my desire and effort

to bring zero cost to consumer information

about science and science-related tools

to the general public.

In keeping with that theme,

I’d like to thank the sponsors of today’s podcast.

Our first sponsor is Athletic Greens.

Athletic Greens is an all-in-one

vitamin mineral probiotic drink.

I’ve been taking Athletic Greens since 2012,

so I’m delighted that they’re sponsoring the podcast.

The reason I started taking Athletic Greens

and the reason I still take Athletic Greens

once or twice a day is that it helps me cover

all of my basic nutritional needs.

It makes up for any deficiencies that I might have.

In addition, it has probiotics,

which are vital for microbiome health.

I’ve done a couple of episodes now

on the so-called gut microbiome

and the ways in which the microbiome interacts

with your immune system, with your brain to regulate mood,

and essentially with every biological system

relevant to health throughout your brain and body.

With Athletic Greens, I get the vitamins I need,

the minerals I need, and the probiotics

to support my microbiome.

If you’d like to try Athletic Greens,

you can go to slash Huberman

and claim a special offer.

They’ll give you five free travel packs

plus a year supply of vitamin D3 K2.

There are a ton of data now showing that vitamin D3

is essential for various aspects

of our brain and body health.

Even if we’re getting a lot of sunshine,

many of us are still deficient in vitamin D3.

And K2 is also important because it regulates things

like cardiovascular function, calcium in the body,

and so on.

Again, go to slash Huberman

to claim the special offer of the five free travel packs

and the year supply of vitamin D3 K2.

Today’s episode is also brought to us by Element.

Element is an electrolyte drink that has everything

you need and nothing you don’t.

That means the exact ratios of electrolytes are an element,

and those are sodium, magnesium, and potassium,

but it has no sugar.

I’ve talked many times before on this podcast

about the key role of hydration and electrolytes

for nerve cell function, neuron function,

as well as the function of all the cells

and all the tissues and organ systems of the body.

If we have sodium, magnesium, and potassium

present in the proper ratios,

all of those cells function properly

and all our bodily systems can be optimized.

If the electrolytes are not present,

and if hydration is low,

we simply can’t think as well as we would otherwise,

our mood is off, hormone systems go off,

our ability to get into physical action,

to engage in endurance and strength

and all sorts of other things is diminished.

So with Element, you can make sure

that you’re staying on top of your hydration

and that you’re getting the proper ratios of electrolytes.

If you’d like to try Element, you can go to drinkelement,

that’s slash Huberman,

and you’ll get a free Element sample pack

with your purchase.

They’re all delicious.

So again, if you want to try Element,

you can go to slash Huberman.

Today’s episode is also brought to us by Thesis.

Thesis makes what are called nootropics,

which means smart drugs.

Now, to be honest, I am not a fan of the term nootropics.

I don’t believe in smart drugs in the sense that

I don’t believe that there’s any one substance

or collection of substances that can make us smarter.

I do believe based on science, however,

that there are particular neural circuits

and brain functions that allow us to be more focused,

more alert, access creativity, be more motivated, et cetera.

That’s just the way that the brain works,

different neural circuits for different brain states.

Thesis understands this.

And as far as I know, they’re the first nootropics company

to create targeted nootropics for specific outcomes.

I’ve been using Thesis for more than six months now,

and I can confidently say that their nootropics

have been a total game changer.

My go-to formula is the clarity formula,

or sometimes I’ll use their energy formula before training.

To get your own personalized nootropic starter kit,

go online to slash Huberman,

take a three-minute quiz,

and Thesis will send you four different formulas

to try in your first month.

That’s slash Huberman,

and use the code Huberman at checkout

for 10% off your first order.

I’m pleased to announce that the Huberman Lab Podcast

is now partnered with Momentus Supplements.

We partnered with Momentus for several important reasons.

First of all, they ship internationally

because we know that many of you are located

outside of the United States.

Second of all, and perhaps most important,

the quality of their supplements is second to none,

both in terms of purity and precision

of the amounts of the ingredients.

Third, we’ve really emphasized supplements

that are single-ingredient supplements

and that are supplied in dosages

that allow you to build a supplementation protocol

that’s optimized for cost,

that’s optimized for effectiveness,

and that you can add things and remove things

from your protocol in a way

that’s really systematic and scientific.

If you’d like to see the supplements

that we partner with Momentus on,

you can go to slash Huberman.

There you’ll see those supplements,

and just keep in mind that we are constantly expanding

the library of supplements available through Momentus

on a regular basis.

Again, that’s slash Huberman.

Okay, let’s talk about light.

First, I want to talk about the physics of light,

and I promise to make that very clear,

even if you don’t have a background in physics.

And then I want to talk about the biology of light,

meaning how light is converted into signals

that your brain and body can use

to impact things like organ health or disease,

or how you can use light

in order to repair particular organs,

like your skin, your eyes, your brain, et cetera.

The physics of light can be made very simple

by just illustrating a few key bullet points.

The first bullet point

is that light is electromagnetic energy.

If the word electromagnetic feels daunting to you,

well then just discard that

and just think of light as energy.

And think of energy as something

that can impact other things in its environment.

Now, the way to imagine light

or to conceptualize light as energy

is that all around you,

light is traveling in these little wavelengths.

And the reason, for those of you that are watching,

I’m making a little wavy motion with my hand,

is that’s actually the way

that light energy moves in little waves.

Just like sound waves are coming at you

and impinging on your ears.

If you can hear me talking right now, that is happening.

Those are sound waves,

meaning the movement of air particles out there

impacting your eardrum.

Well, light energy is just little bits

of electromagnetic energy

traveling through your environment all the time

in these little waves

and impinging on your brain and body and eyes, et cetera.

And as I mentioned before,

energy can change the way that other things behave.

It can cause reactions in cells of your body.

It can cause reactions in fruit, for instance, right?

You see a piece of fruit and it’s not ripe,

but it gets a lot of sunlight and it ripens.

That’s because the electromagnetic energy of sunlight

had an impact on that plant or that tree

or even on the fruit directly.

As a parallel example of energy

and its ability to impact other things,

we are all familiar with food

and the fact that food has calories.

Calorie is a measure of energy.

It has everything to do with how much heat is generated

when you burn a particular article of food,

believe it or not.

And it turns out that how hot a given article of food burns

gives you a sense of how much energy

it can provide your body

in terms of your body’s ability to store

or use that energy.

So again, think of light as electromagnetic energy,

but really put that word energy into capital letters.

Embed that in your mind going forward

and you’ll understand most of the first bullet point

of what light is in terms of the physics of light.

Now, the second thing that you need to understand

about the physics of light

is that light has many different wavelengths.

And the simplest way to conceptualize this

is to imagine that cover of that Pink Floyd album

where there’s a prism,

you have a white beam of light going into that prism,

and then the prism splits that beam of light

into what looks like a rainbow.

So you’ve got your reds, your orange, your greens,

your blues, your purples, et cetera.

Anytime we have light in our environment

that is so-called white light,

it includes all those wavelengths,

but sunlight and other forms of light

also have other wavelengths of light that we can’t see.

So when we think about the rainbow,

that’s just the visible spectrum of light.

There are also wavelengths of light

that are not visible to us,

but that are visible to some other animals

and that can still impact your brain and body

because there is still energy at those wavelengths.

I’ll give a few examples of this.

Humans are not a species that can see

into the infrared realm of the spectrum.

A pit viper, meaning a snake that has infrared sensors,

however, can sense in the infrared.

So if you were to walk through a jungle

and there’s a pit viper there,

it sees you as a cloud of heat emission

because your body is emitting infrared energy all the time.

You’re casting off infrared energy.

The snake can see it, you can’t.

If you were to put on a particular set of goggles

that were infrared goggles,

well, then you would be able to see the heat emissions

of any organism, human or otherwise,

that could emit infrared energy.

Let’s take the opposite end of the spectrum.

We are familiar with seeing things that are blue or green

or very pale blue, but as we say below that,

meaning even shorter wavelength light is out there.

Ultraviolet light is a really good example

of light energy that’s coming from the sun

and is in our environment

and is being reflected off surfaces all the time.

We don’t see it.

And yet if it’s very bright outside,

that ultraviolet light can burn our skin.

As you’ll learn in today’s episode,

ultraviolet light can also positively impact us.

In fact, I will describe a particular set of new results

that show that ultraviolet light

viewed for just a few minutes each day

or landing on the skin for just a few minutes each day

can actually offset a lot of pain.

It actually has the ability to reduce the amount of pain

sensed by your body.

And we now understand the specific circuits

in the brain and body that allow that to happen.

I’ll talk about that

and the related protocols a little bit later.

So the important thing to understand

about the physics of light

is that there’s energy at all these different wavelengths.

We only see some of those wavelengths,

which basically is to say that light impacts us

at many different levels.

And the so-called levels that I’m referring to

are the different wavelengths of light.

And you’re welcome to think of the different wavelengths

of light as different colors,

but do understand that there are truly colors of light

that you and I can’t see,

and yet that have powerful impact on your brain and body.

Now, the third bullet point to understand

about the physics of light

is that different wavelengths of light,

because of the way that their wave travels

can penetrate tissues to different depths.

This is very, very important.

Today, we’re going to talk a lot about red light therapies

and near infrared light therapies.

Those are so-called longer wavelengths.

Longer wavelength, just think of a bigger, longer wave,

right, a bigger curve,

as opposed to short wavelength light,

which is going to be shorter, right?

A short wavelength light would be something like blue

or green light or ultraviolet light.

Blue, green, and ultraviolet light,

because it’s short wavelength light,

doesn’t tend to penetrate tissues very easily.

It has to do with the way that the physics of light

interacts with the physical properties of your skin

and other tissues of your body.

But basically, if you were to shine UV light

onto your arm, for instance,

it could impact the skin on the surface of the arm

and maybe some of the cells

just beneath the top layer of skin,

but it wouldn’t penetrate much deeper.

Long wavelength light, like red light

and near infrared light,

has this amazing ability to penetrate through tissues,

including your skin.

And so if we were to shine red light

or near infrared light onto your arm,

it would pass through that top layer of skin.

It might impact it a little bit,

but it could penetrate deeper into your skin,

not just to the skin layers,

but maybe even down to the bone,

maybe even down to the bone marrow.

And for many people,

this will be hard to conceptualize.

You think, well, wait, I’ve got a skin there.

Doesn’t the light just bounce off?

And the answer is no,

because of the way that long wavelength light

interacts with the absorbance properties of your skin.

Absorbance properties is just the way

that the skin takes light energy

and converts it into a different form of energy.

And your skin is not able to take long wavelength light,

like red light and near infrared light, and absorb it,

but the tissues deeper in your body can.

So if you shine red light or near infrared light

onto the surface of your skin,

you’ll see a red glow there

as a reflectance on the surface of your skin.

But a lot of the photon energy,

the light energy in those longer wavelengths

is indeed passing through those top layers of skin

into the deeper layers of skin,

and can even make it into the deep layers of your arm.

And as we start to transition from the physics of light

to the biological impacts of light,

just understanding that the different wavelengths of light

impact our tissues at different levels,

literally at different depths,

will help you better understand

how light of different colors, of different intensities,

and how long you’re exposed to those colors

and intensities of light can change the way

that the cells and the organs of your body work.

And if it didn’t sound weird enough

that you can pass light through particular tissues

and have them land and be absorbed

at tissues deeper in your body,

well, it turns out that different wavelengths of light

are also best absorbed by particular

so-called organelles within your cells.

What are organelles?

Organelles are the different compartments

and different functions within a given cell.

So for instance, your mitochondria,

which are responsible for generating ATP

and energy in your cells.

Those exist at a particular depth,

at a particular location within a cell.

They’re not all at the cell surface,

they sit somewhat deeper in the cell.

The nucleus of your individual cells contains DNA,

and that sits at a particular depth

or location within your cell.

Different wavelengths of light,

not only can penetrate down into different tissues

and into different cells of your body,

but they can also penetrate and access

particular organelles, meaning mitochondria,

or the nucleus, or the different aspects of your cells

that are responsible for different functions.

This is exquisitely important,

and it’s exquisitely powerful,

because as you’ll learn today,

particular wavelengths of light can be used

to stimulate the function of particular organelles

within particular cells,

within particular organs of your body.

I can think of no other form of energy,

not sound, not chemical energy, so not drugs,

not food, not touch,

no form of energy that can target

the particular locations in our cells,

in our organelles, in our organs, and in our body,

to the extent that light can.

In other words, if you had to imagine

a real-world surgical tool

by which to modulate our biology,

light would be the sharpest

and the most precise of those tools.

Now let’s talk about how light is converted

into biological signals.

There are several ways in which that is accomplished,

but the fundamental thing to understand

is this notion of absorption of light energy.

Certain pigments or colors

in the thing that is receiving the light energy,

meaning the thing that the light energy lands on,

are going to absorb particular wavelengths of light.

Now, I promise you

that you already intuitively know how this works.

For instance, if you were to sit outside

on a very bright, sunny day,

and you had a table in front of you that was metal,

you might find it hard to look down at that metal table,

because it’s reflecting a lot of light

of particular wavelengths.

If that table were pitch black, however,

it wouldn’t reflect quite as much,

and you would be able to comfortably look at it.

If that table were red, it might be somewhere in between.

If that table were green,

it would be also somewhere in between.

But let’s say it were very light blue.

Well, then it might reflect almost as much

as a table that were just metal or a white table surface.

So the absorbance properties of a given surface

will determine whether or not light energy goes

and stays at that location

and has an impact on that location,

or whether or not it bounces off.

Every biological function of light

has to do with the absorbance or the reflectance of light

or light passing through that particular thing,

meaning that particular cell or compartment within a cell.

I’d like to make it clear how this works

by using the three primary examples

of how you take light in your environment

and convert it into biological events.

We have photoreceptors in the back of our eyes.

These photoreceptors come in two major types,

the so-called rods and the cones.

The rods are very elongated.

They look like rods,

and the cones look like little triangles.

Rods and cones have within them photopigment.

They have dark stuff that’s stacked up in little layers.

Rods absorb light of essentially any wavelength.

There’s some variation to that,

but let’s just say rods don’t care

about the different colors of light.

They will absorb light energy, photon energy.

If it’s red, if it’s green, if it’s blue, if it’s yellow,

doesn’t matter as long as that light is bright enough.

And it turns out that rods are very, very sensitive.

They can detect very, very small numbers of photons.

And rods are essentially what you use to see

in very low light conditions.

We’ll return more to vision later.

The cones come in three major varieties,

at least for most people who aren’t colorblind.

You have so-called red cones, green cones, and blue cones,

but they’re not really red, green, and blue

in the back of your eye.

They are cones that either absorb long wavelength light,

red, that absorb medium wavelength light, green,

or short wavelength light, blue.

The reason that they can absorb

different wavelengths of light

is they have different photo pigments.

So much as the example I gave before,

where you have different tables outside

in the sunny environment,

and some are reflecting light more than others,

others are absorbing light more than others.

Well, so too, the photoreceptors, meaning the cones,

are absorbing light of different wavelengths

to different extents.

And in a absolutely incredible way,

your brain is actually able to take that information

and create this perception that we have of color.

But that’s another story altogether

that we’ll just touch on a little bit more later,

but that if you want to learn all about,

you can go to our episode on vision.

So that’s photoreceptors in the back of your eye

absorbing light of different wavelengths, rods and cones.

The other place, of course,

where light can impact our body is on our surface,

on our skin, and skin has pigment too.

We call that pigment melanin.

We have within our skin multiple cell types,

but in the top layer of skin,

which is called the epidermis,

we have keratinocytes and we have melanocytes.

And the melanocytes are the cells

that create pigmentation of the skin.

And of course, there is wide variation

in the degree to which there’s pigmentation of the skin,

which has to do with genetics,

also has to do with where you were born and raised,

how much light exposure you have throughout the year, right?

So people toward the equator

tend to have more melanocyte activity

than people who are located at the North Pole.

And of course, people live at different locations

throughout the earth, regardless of their genetic background

or where they were born.

And so, as you all know, with light exposure,

those melanocytes will turn on genetic programs

and other biological programs

that lead to enhanced pigmentation of the skin,

which we call tanning.

The way they do that is by absorbing UV light specifically.

So with melanocytes, we have a very specific example

of how a pigment absorbs light of a particular length,

in this case, ultraviolet short wavelength light,

which in turn creates a set of biological signals

within those cells that in turn

creates changes in our skin pigmentation.

So we have photoreceptors, we have melanocytes.

And the third example I’d like to provide

is that of every cell of your body.

And what I mean by that is that every cell of your body,

meaning a cell that is part of your bone tissue

or your bone marrow or heart tissue or liver or spleen,

if light can access those cells,

it will change the way that those cells function

for better or for worse.

For many organs within our body

that reside deep to our skin,

light never arrives at those cells.

A really good example of this that we’ll touch on later

is the spleen.

Unless you have massive damage to your body surface,

unless you literally have a hole in your body,

light will never land directly on your spleen.

But the spleen still responds to light information

through indirect pathways.

And those indirect pathways arise

through light arriving on the skin

and light arriving on the eyes.

So a key principle that I’m going to return to again

and again today is that the ways in which light

can impact the biology of your organelles, your cells,

your organs and the tissues and indeed your whole body

can either be direct.

So for instance, light onto your skin impacting skin

or light onto your photoreceptors

impacting the photoreceptors of your eye,

or it can be indirect.

It can be light arriving on your photoreceptors,

the photoreceptors then informing another cell type,

which informs another cell type,

which then relays a signal in kind of a bucket brigade

manner off to the spleen and says to the spleen,

hey, there’s a lot of UV light out here.

We’re actually under stress.

In fact, there’s so much UV light that you need to activate

an immune program to protect the skin.

And in response to that,

the spleen can deploy certain signals in certain cell types

to go out and start repairing skin

that’s being damaged by UV light.

So we have direct signals and we have indirect signals,

but in every case,

it starts with light of particular wavelengths

being absorbed by particular pigments

or properties of the surfaces

that those light waves land on.

And as you recall from our discussion

about the physics of light,

remember it’s not just about light impinging

on the surface of your body.

Light can actually penetrate deep to the skin

and access at least certain tissues and cells of your body.

Even though you can’t see those wavelengths of light,

they are getting into you all the time.

So perhaps the best way to wrap this discussion

about the physics and the biology of light

with a bit of a bow is to think about light as a transducer,

meaning a communicator of what’s going on

in the environment around you.

And that some of those signals are arriving at the surface

and impacting the surface of your body.

But many of those signals are being taken by cells

at the surface of your body,

meaning your melanocytes in your skin

and the photoreceptors of your eyes,

and then being passed off as a set of instructions

to the other organs and tissues of your body.

Light can impact our biology in very fast,

moderately fast and slow ways.

But even the slow ways in which light can impact our biology

can be very powerful and very long lasting.

Just as a quick example of the rapid effects of light

on our biology.

If you were to go from a room that is dimly lit or dark

into a very brightly lit room,

you would immediately feel very alert.

You might say, no, that’s not true.

Sometimes I wake up and it’s dark and I kind of stumble out

and it’s lighter out in the next room

and it takes me a while to wake up.

But if we were to move you from a room that was very dark

to very bright, a signal conveyed from your eyes

to an area of your brainstem called the locus coeruleus

would cause the release of adrenaline,

similar to the release of adrenaline

if you were to be dropped into very, very cold water

all of a sudden.

It’s just an immediate wake up signal

to your brain and body.

So that’s an example of a rapid effect of light

on your biology.

Not a very typical one, but nonetheless,

one that has a hardwired biological mechanism.

At the other end of the spectrum are what we call

slow integrating effects of light on our biology.

So what I mean by that are ways in which your body

is taking information about light in the environment,

not in the sort of snapshot acute sense,

but averaging the amount of light in your environment.

And that average light information is changing the way

that your biology works.

But even though this is a slow process,

as I mentioned before, it’s a very powerful one.

The primary example of this are so-called

circanual rhythms.

Circanual rhythms are literally a calendar

that exists within your body that uses not numbers,

but amounts of hormone that are released

into your brain and body each day and each night

as a way of knowing where you are

in the 365 day calendar year.

Now that might seem kind of crazy, but it’s not crazy.

The earth travels around the sun once every 365 days.

And depending on where you are on the earth,

where you live, you are going to get more or less light

each day on average, depending on the time of year.

So if you’re in the Northern hemisphere

in the winter months, days are shorter, nights are longer.

In the summer months, days are longer, nights are shorter.

And of course things change whether or not

you’re in the Northern hemisphere

or the Southern hemisphere.

But nonetheless, in short days, you have more darkness.

That’s obvious.

And if you understand that light arriving on the eyes

is absorbed by a particular cell type

called the intrinsically photosensitive ganglion cell.

It’s just a name.

You don’t need to know the name, but if you want,

it’s the so-called intrinsically photosensitive ganglion cell

also called the melanopsin cell

because it contains an opsin, a photo pigment

that absorbs short wavelength light

that arrives through sunlight.

Those cells communicate to particular stations in the brain

that in turn connect to your so-called pineal gland,

which is this little pea-sized gland

in the middle of your brain that releases a hormone

called melatonin.

And the only thing you need to know

is that light activates these particular cells,

the intrinsically photosensitive melanopsin cells,

which in turn shuts down the production of melatonin

from the pineal gland.

If you think about this in terms of the travel of the earth

around the sun across the year,

what it means is that in short days,

because there’s very little light on average

landing on these cells,

the duration of melatonin release will be much longer

because as I mentioned before, light inhibits,

it shuts down melatonin.

Whereas in the summer months,

much more light on average will land on your eyes, right?

Because days are longer,

even if you’re spending more time indoors,

on average, you’re going to get more light

to activate these cells.

And because light shuts down melatonin production,

what you’ll find is that the duration of melatonin release

for the pineal is much shorter.

So melatonin is a transducer.

It’s a communicator of how much light on average

is in your physical environment.

What this means is for people living

in the Northern hemisphere,

you’re getting more melatonin release in the winter months

than you are in the summer months.

So you have a calendar system that is based in a hormone

and that hormone is using light

in order to determine where you are

in that journey around the sun.

Now, this is beautiful.

At least to me, it’s beautiful

because what it means is that the environment around us

is converted into a signal

that changes the environment within us.

That signal is melatonin.

And melatonin is well-known for its role

in making us sleepy each night

and allowing us to fall asleep.

Many of you have probably heard before,

I am not a big fan of melatonin supplementation

for a number of reasons, but just as a quick aside,

the levels of melatonin that are in most supplements

are far too high to really be considered physiological.

They are indeed super physiological in most cases.

And melatonin can have a number of different effects,

not just related to sleep,

but that’s supplemented melatonin.

Here, I’m talking about our natural production

and release of melatonin

according to where we are in the 365-day calendar year.

Endogenous melatonin,

meaning the melatonin that we make

within our bodies naturally,

not melatonin that’s supplemented,

has two general categories of effects.

The first set of effects are so-called regulatory effects,

and the others are protective effects.

The regulatory effects are, for instance,

that melatonin can positively impact bone mass.

So melatonin can, for instance,

turn on the production of osteoblasts,

which are essentially stem cells

that make more bone for us,

that make our bones stronger

and that can replace damaged aspects of our bone.

Melatonin is also involved in maturation of the gonads

during puberty, the ovaries and the testes.

Although there, the effects of melatonin

tend to be suppressive on maturation

of the ovaries and testes,

meaning high levels of melatonin

tend to reduce testicle volume

and reduce certain functions within the testes,

including sperm production and testosterone production.

And within the ovaries,

melatonin can suppress the maturation of eggs, et cetera.

Now, I don’t want anyone to get scared

if you’ve been taking melatonin.

Most of the effects of melatonin on those functions

are reversible, but I should point out

that one of the reasons why children

don’t go into puberty until a particular age

is that young children tend to have

chronically high endogenous melatonin,

and that is healthy to keep them out of puberty

until it’s the right time for puberty to happen.

So melatonin can increase bone mass,

but reduces gonad mass, so to speak.

It’s going to have varying effects

depending on the ratios and levels of other hormones

and other biological events in the body.

But as you can see,

melatonin has these powerful regulatory effects

on other tissues.

I should also mention that melatonin

is a powerful modulator of placental development.

So for anyone that’s pregnant,

if you’re considering melatonin supplementation,

please, please, please talk to your OBGYN,

talk to your other doctor as well.

You want to be very, very cautious

because of the powerful effects that melatonin can have

on the developing fetus and placenta.

For people that are not pregnant,

in fact, all people,

melatonin has a powerful effect

on the central nervous system as a whole.

Your brain and spinal cord are the major components

of your central nervous system.

And melatonin, because it’s associated with darkness,

which is just another way of saying

that light suppresses melatonin,

melatonin is thereby associated

with the dark phase of each 24-hour cycle.

It can have a number of different effects

in terms of waking up or making our body feel more sleepy.

And it does that by way of impacting cells

within our nervous system,

literally turning on certain brain areas,

turning off other brain areas.

And it does that

through a whole cascade of biological mechanisms,

a bit too detailed to get into today.

So melatonin is regulating how awake or asleep we are.

It tends to make us more asleep incidentally.

It’s regulating our timing of puberty

and it’s regulating how our gonads,

the testes and ovaries function,

even in adulthood to some extent.

And it’s regulating bone mass.

As I mentioned before,

melatonin also has protective effects.

It can activate our immune system.

It is among the most potent antioxidants.

So it is known to have certain anti-cancer properties

and things of that sort,

which is not to say that you simply want more melatonin.

I think a lot of people get misled

when they hear something

like melatonin has anti-cancer properties.

That doesn’t mean that just cranking up the levels

of melatonin by supplementing it

or by spending time in darkness and not getting any light,

which would of course inhibit melatonin,

is going to be beneficial for combating cancer.

That’s not the way it works.

It is actually the rise and fall of melatonin

every 24 hour cycle.

And the changes in the duration of that melatonin signal

throughout the seasons

that has these anti-cancer and antioxidant effects.

So when we think about light impacting our biology,

the reason I bring up melatonin

as the primary example of that

is A, because melatonin impacts

so many important functions within our brain and body,

but also because hormones in general, not always,

but in general are responsible

for these slow modulatory effects on our biology.

And so I’m using this as an example

of how light throughout the year

is changing the way that the different cells and tissues

and organs of your body are working

and that melatonin is the transducer of that signal.

So at this point we can say

light powerfully modulates melatonin,

meaning it shuts down melatonin.

Melatonin is both beneficial for certain tissues

and suppressive for other tissues and functions.

What should we do with this information?

Well, it’s very well established now

that one of the best things that we can all do

is to get the proper amount of sunlight each day.

And by proper, I mean appropriate for that time of year.

So in the summer months where the days are longer

and nights are shorter,

we would all do well to get more sunlight in our eyes.

And again, it’s going to be to our eyes

because as you recall, the pineal sits deep in the brain

and light can’t access the pineal directly,

at least not in humans.

So in order to get light information to the pineal

and thereby get the proper levels of melatonin,

according to the time of year,

we should all try and get outside as much as possible

during the long days of summer and spring.

And in the winter months,

it makes sense to spend more time indoors.

For those of you that suffer

from seasonal affective disorder,

which is a seasonal depression

or feel low during the fall and winter months,

there are ways to offset this.

We did an entire episode on mood and circadian rhythms

where we described this.

So it does make sense for some people

to get more bright light in their eyes early in the morning

and throughout the day during the winter months as well.

But nonetheless, changes in melatonin,

meaning changes in the duration of melatonin release

across the year are normal and healthy.

So provided that you’re not suffering from depression,

it’s going to be healthy to somewhat modulate your amount

of indoor and outdoor time across the year.

The other thing to understand

is this very firmly established fact,

which is light powerfully inhibits melatonin.

If you wake up in the middle of the night

and you go into the bathroom

and you flip on the lights

and those are very bright overhead fluorescent lights,

your melatonin levels,

which would ordinarily be quite high

in the middle of the night

because you’ve been eyes closed in the dark, presumably,

will immediately plummet to near zero or zero.

We would all do well, regardless of time of year,

to not destroy our melatonin

in the middle of the night in this way.

So if you need to get up in the middle of the night

and use the restroom,

which is a perfectly normal behavior for many people,

use the minimum amount of light required

in order to safely move through the environment

that you need to move through.

Melatonin needs to come on early in the night.

It actually starts rising in the evening and towards sleep.

But then as you close your eyes and you go to sleep,

melatonin levels are going to continue to rise,

at least for several hours into the night.

Again, if you get up in the middle of the night,

really try hard not to flip on a lot of bright lights.

If you do that every once in a while,

that’s not going to be a problem.

But if you’re doing that night after night,

you are really disrupting this fundamental signal

that occurs every night,

regardless of winter, spring, summer, et cetera,

and that is communicating information

about where your brain and body should be in time.

And I know that’s a little bit of a tricky concept,

but really our body is not meant to function

in the same way during the winter months

as the summer months.

There are functions that are specifically optimal

for the shorter days of winter,

and there are functions that are specifically optimal

for the longer days of summer.

So again, try to avoid bright light exposure to your eyes

in the middle of the night.

And for those of you that are doing shift work,

what I can say is try and avoid getting bright light

in your eyes in the middle of your sleep cycle.

So even if you’re sleeping in the middle of the day

because you have to work at night,

if you wake up during that bout of sleep,

really try hard to limit the amount of light,

which is going to be harder for shift workers, right?

Because there are generally a lot more lights on

than bright lights outside to you.

You would want to close the blinds

and limit artificial light inside.

One way to bypass some of the inhibitory effects of light

on melatonin is to change your physical environment

by, for instance, dimming the lights.

That’s one simple way, very low cost way.

In fact, you’ll save money by dimming the lights

or turning them off.

The other is if you are going to use light

using long wavelength light, because as you recall,

these intrinsically photosensitive melanopsin cells

within your retina that convey the signal

about bright light in your environment to impact melatonin,

to shut down melatonin,

respond to short wavelengths of light.

So red light is long wavelength light.

You now understand that from our discussion

about the physics of light.

And if you were to use amber colored light or red light

and even better dim amber or dim red light

in the middle of the night,

well then you would probably not reduce melatonin at all,

unless those red lights and amber lights

are very, very bright.

Any light provided it’s bright enough

will shut down melatonin production.

One final point about melatonin,

and this relates to melatonin supplementation as well,

is that now that you understand how potently melatonin

can impact things like cardiovascular function,

immune function, anti-cancer properties, bone mass,

gonad function, et cetera,

you can understand why it would make sense to be cautious

about melatonin supplementation,

because supplementation tends to be pretty static.

It’s X number of milligrams per night.

Whereas normally, endogenously,

the amount of melatonin that you’re releasing each night

is changing according to time of year.

Or if you happen to live in an area

where there isn’t much change in day length across the year,

so for instance, if you live near the equator,

well, then your body is accustomed

to having regular amounts of melatonin each night.

When you start supplementing melatonin,

you start changing the total amount of melatonin, obviously,

but you’re also changing the normal rhythms

in how much melatonin is being released

into your brain and body across the 365-day calendar year.

So while I’m somebody who readily embraces supplementation

in various forms for things like sleep and focus, et cetera,

when it comes to melatonin, I’m extremely cautious.

And I think it’s also one of the few examples

where a hormone is available without prescription


You can just go into a pharmacy or drugstore,

order online this hormone,

which is known to have all these powerful effects.

So I get very, very concerned

when I hear about people taking melatonin,

especially at the levels that are present

in most supplements.

It’s been recognized for a very long time,

and in fact, there are now data to support the fact

that animals of all kinds, including humans,

will seek out mates and engage in mating behavior

more frequently during the long days of spring and summer.

That’s right.

In seasonally breeding animals, of course this is the case,

but in humans as well,

there is more seeking out of mates and mating behavior

in longer day times of year.

Now you could imagine at least two mechanisms

by which this occurs.

The first mechanism we could easily map to melatonin

and the fact that melatonin is suppressive

to various aspects of the so-called gonadal axis,

which is basically a fancy way of saying

that melatonin inhibits testosterone and estrogen output

from the testes and from the ovaries.

I just want to remind people that both males and females

make testosterone and estrogen,

although in different ratios,

typically in males versus females,

and that both testosterone and estrogen are critical

for the desire to mate and for mating behavior.

There’s a broad misconception

that testosterone is involved in mating behavior

and estrogen is involved in other behaviors,

but having enough estrogen is critical

for both males and females in order to maintain

the desire to mate and indeed the ability to mate.

I discussed this on the episode

on optimizing testosterone and estrogen,

so if you’d like more details on that,

please see that episode of the Huberman Lab Podcast.

Okay, so if melatonin is suppressive

to the so-called gonadal axis

and reduces overall levels of testosterone and estrogen

in males and females, and light inhibits melatonin,

then when there’s more light, then there’s less melatonin

and more hormone output from the gonads.

And indeed, that’s how the system works,

but that’s not the entire story.

It turns out that there is a second

so-called parallel pathway,

meaning a different biological pathway

that operates in parallel to the light suppression

of melatonin pathway that provides a basis

for longer days inspiring more desire to mate

and more mating behavior.

So if we think of the first pathway involving melatonin

as sort of a break on these reproductive hormones,

the second mechanism is more like an accelerator

on those hormones, and yet it still involves light.

As I’m about to tell you, in animals such as mice,

but also in humans, exposure to light,

in particular UV blue light, so short wavelengths of light,

can trigger increases in testosterone and estrogen

and the desire to mate.

Now, what’s especially important about this accelerator

on the desire to mate and mating behavior and hormones

is that it is driven by exposure to light,

but it is not the exposure of light to the eyes.

It turns out that it is the exposure of your skin

to particular wavelengths of light

that is triggering increases in the hormones,

testosterone and estrogen,

leading to increased desire to mate.

As it turns out, your skin,

which most of us just think of as a way

to protect the organs of our body

or something to hang clothes on or ornaments on

if you’re somebody who has earrings and so forth,

your skin is actually an endocrine organ,

meaning it is a hormone producing

and hormone influencing organ.

I promise what I’m about to tell you next

will forever change the way that you think about your skin

and light and the desire to mate

and indeed even mating behavior.

I think the results are best understood

by simply going through the primary data,

meaning the actual research on this topic.

And to do so, I’m going to review a recent paper

that was published in the journal Cell Reports,

Cell Press Journal, excellent journal.

This is a paper that came out in 2021

entitled Skin Exposure to UVB Light

Induces a Skin-Brain-Gonad Axis and Sexual Behavior.

And I want to emphasize that this was a paper

that focused on mice in order to address

specific mechanisms because in mice,

you can so-called knock out particular genes,

you can remove particular genes to understand mechanism.

You just can’t do that in humans

in any kind of controlled way,

at least not at this point in time.

And this study also explores humans

and looked at human subjects, both men and women.

The basic finding of this study was that

when mice or humans were exposed to UVB,

meaning ultraviolet blue light,

so short wavelength light of the sort

that comes through in sunshine,

but is also available through various artificial sources.

If they received enough exposure

of that light to their skin,

there were increases in testosterone

that were observed within a very brief period of time,

also increases in the hormone estrogen.

And I should point out that the proper ratios

of estrogen and testosterone were maintained

in both males and females,

at least as far as these data indicate.

And mice tended to seek out mating more and mate more.

There were also increases in gonadal weight,

literally increases in testes size and in ovarian size

when mice were exposed to this UVB light

past a certain threshold.

Now, as I mentioned before, the study also looked at humans.

They did not look at testes size

or ovarian size in the human subjects.

However, because they are humans,

they did address the psychology of these human beings

and address whether or not they had increases in,

for instance, aggressiveness or impassionate feelings

and how their perception of other people changed

when they were getting a lot of UVB light exposure

to the skin.

So before I get into some of the more important details

of the study and how it was done

and how you can leverage this information for yourself,

if you desire, I just want to highlight

some of the basic findings overall.

UVB exposure increased these so-called sex steroid levels

in mice and humans.

The sex steroid hormones, when we say steroids,

we don’t mean anabolic steroids taken exogenously.

I think when people hear the word steroids,

they always think steroid abuse or use,

rather steroid hormones such as testosterone and estrogen

went up when mice or humans had a lot of UVB exposure

to their skin.

Second of all, UVB light exposure to the skin

enhanced female attractiveness.

So the perceived attractiveness of females by males

and increase the receptiveness or the desire to mate

in both sexes.

UVB light exposure also changed various aspects

of female biology related to fertility,

in particular, follicle growth.

Follicle and egg maturation are well-known indices

of fertility and of course correlate

with the menstrual cycle in adult humans

and is related overall to the propensity to become pregnant.

UVB light exposure enhanced maturation of the follicle,

which just meant that more healthy eggs were being produced.

These are impressive effects.

First of all, they looked at a large number of variables

in the study and the fact that they looked at mice

and humans is terrific.

I think that oftentimes we find it hard to translate data

from mice to humans.

So the fact that they looked at both in parallel

is wonderful.

In the mice and in the humans,

they established a protocol that essentially involved

exposing the skin to UV light that was equivalent

to about 20 to 30 minutes of midday sun exposure.

Now, of course, where you live in the world

will dictate whether or not that midday sun

is very, very bright and intense or is less bright.

Maybe there’s cloud cover, et cetera.

But since I’m imagining that most people are interested

in ways to increase testosterone and or estrogen

in humans and are not so much interested

in increasing testosterone in mice,

I’m going to just review what they did

in the human population or the human subjects.

What they did is they had people,

first of all, establish a baseline.

And the way they establish a baseline

was a little bit unusual,

but it will make perfect sense to you.

They had people wear long sleeves and essentially cover up

and avoid sunlight for a few days

so they could measure their baseline hormones

in the absence of getting a lot of UVB light exposure

from the sun or from other sources.

Now, of course, these people had access

to artificial lights,

but as I’ve pointed out on this podcast before,

it’s pretty unusual that you’ll get enough UVB exposure

from artificial lights throughout the day.

And in the morning, you need a lot of UVB exposure,

or we should be getting a lot of UVB exposure

to our eyes and to our face and to our skin

throughout the day, provided we’re not getting sunburned.

This is actually a healthy thing for mood

and for energy throughout the day.

It’s only at night, basically between the hours

of about 10 p.m. and 4 a.m.

that even a tiny bit of UVB exposure

from artificial sources can mess us up

in terms of our sleep and our energy levels and so on.

And that’s because of the potent effect of UVB

on suppressing melatonin.

So the point here is that they establish a baseline

whereby people were getting some artificial light exposure

throughout the day, but they weren’t getting outside a lot.

They weren’t getting a lot of sunlight.

And then they had people receive a dose

of UVB light exposure

that was about 20 to 30 minutes outdoors.

They had people wear short sleeves, no hat, no sunglasses.

Some people wore sleeveless shirts.

They encouraged people to wear shorts.

So they were indeed wearing clothing.

They were not naked and they were wearing clothing

that was culturally and situationally appropriate,

at least for the part of the world

where this study was done.

And they had people do that two or three times a week.

So in terms of a protocol that you might export

from this study, basically getting outside

for about 30 minutes, two or three times a week

in a minimum of clothing,

and yet still wearing enough clothing

that is culturally appropriate.

They were outside, they weren’t sunbathing,

flipping over on their back and front.

They were just moving about doing things they could read,

they could talk, they could go about other activities,

but they weren’t wearing a broad brim hat

or a hat of any kind.

Just getting a lot of sun exposure to their skin.

They did this for a total of 10 to 12 UVB treatments.

So this took several weeks, right?

It took about a month if you think about it,

two or three times per week

for a total of 10 to 12 UVB treatments.

These treatments, of course,

are just being outside in the sun.

And then they measured hormones

and they measured the psychology

of these male and female adult subjects.

Let’s first look at the psychological changes

that these human subjects experienced

after getting 10 to 12 of these UVB light exposure,

outdoor and sunlight type treatments.

They did this by collecting blood samples

throughout the study.

And they saw significant increases

in the hormones beta-estradiol,

which is one of the major forms of estrogen,

progesterone, another important steroid hormone,

and testosterone in both men and women.

Now, an important point is that the testosterone increases

were significantly higher in men

that happened to originate from countries

that had low UV exposure

compared to individuals from countries

with high UV exposure.

Now, this ought to make sense

if we understand a little bit

about how the skin functions as an endocrine organ.

Many of you have probably heard of vitamin D3,

which is a vitamin that we all make.

Many people supplement it as well

if they need additional vitamin D3.

We all require sunlight in order to allow vitamin D3

to be synthesized and perform its roles in the body.

And it turns out that people who have darker skin

actually need more vitamin D3

and or more sunlight exposure

in order to activate that D3 pathway

than do people with paler skin.

And this should make sense to all of you

given what you now understand about melanocytes,

that cell type that we discussed earlier,

because melanocytes have pigment within them.

And if you have darker skin,

it means that you have more melanocytes

or that you have melanocytes

that are more efficient at creating pigment.

And as a consequence,

the light that lands on your skin

will be absorbed by those melanocytes

and less of it is able to impact the D3 pathway.

Whereas if you have pale skin,

more of the light that lands on your skin

can trigger the synthesis

and assist the actions of vitamin D3.

Similarly, in this study,

they found that people who had paler skin

and or who originated from countries

where they had less UVB light exposure across the year

had greater, meaning more significant increases

in testosterone overall

than did people who already

were getting a lot of UVB exposure.

This led them to explore

so-called seasonal changes in testosterone

that occurred normally in the absence

of any light exposure treatment.

So up until now,

I’ve been talking about the aspects of this study

involving people getting outside

for about 20 to 30 minutes per day

in sunlight in a minimum of clothing.

There was an increase in testosterone observed

in both men and women.

The increases in testosterone were greater

for people that had paler skin than darker skin.

So the data I’m about to describe

also come from this same paper,

but do not involve 20 to 30 minute

daily sun exposure protocols.

It’s simply addressing whether or not

testosterone levels change as a function of time of year.

They measure testosterone across the 12 month calendar.

This study was done on subjects living

in the Northern hemisphere for the entire year.

And so in the months of January, February, and March,

of course, the length of days is shortest

and the length of nights is longest.

And of course, in the spring and summer months,

June, July, August, September, and so on,

the days are much longer and the nights are shorter.

And what they observed was very obvious.

They observed that testosterone levels

were lowest in the winter months

and were highest in the months

of June, July, August, and September.

Now, these are very important data.

At least to my knowledge,

these are the first data systematically exploring

the levels of sex steroid hormones in humans

as a function of time of year and thereby

as a function of how much sunlight exposure they’re getting.

And what’s remarkable about these data

is that they map very well to the data in mice

and the other data in this paper on humans,

which illustrate that if you’re getting more UVB exposure,

your testosterone levels are higher.

This study went a step further and explored

whether or not the amount of sunlight exposure

that one is getting to their skin influences

their psychology in terms of whether or not

they have increased desire to mate and so on.

It’s well-known that sunlight exposure

to the eyes can increase mood.

And I talked about this in the podcast episode

with my guest, Dr. Samer Hattar,

who’s the director of the chronobiology unit

at the National Institutes of Mental Health.

And Samer’s recommendation is that people get

as much bright light exposure as they safely can

in the morning and throughout the day

for sake of both sleep and energy,

but also for enhancing mood and regulating appetite.

In this study, it was found that both males and females

had higher levels of romantic passion

after getting the UV treatment.

In fact, some of them reported increases in romantic passion

from just one or two of these UV treatments.

So they didn’t have to go through all 10 or 12

in order to get a statistically significant increase

in passion.

Now, when we talk about passion,

as the authors of this paper acknowledge,

there’s really two forms.

There is emotional and sexual.

And they parsed this pretty finely.

I don’t want to go into all the details

and we can provide a reference and link to this study

if you’d like to look at those details.

But what they found was that women receiving

this UVB light exposure focused more on increases

in physical arousal and sexual passion,

whereas the men actually scored higher

on the cognitive dimensions of passion,

such as obsessive thoughts about their partner and so on.

Regardless, both males and females experienced

and reported a increase in sexual passion

and desire to mate.

And we now know there were increases in testosterone

and estrogen, which of course could be driving

the psychological changes.

Although I’m sure that those interact in both directions,

meaning the hormones no doubt affect psychology

and no doubt the psychology,

these changes in passionate feelings,

no doubt also increased or changed

the hormone levels as well.

And I want to reemphasize that there was a component

of the study that had no deliberate daylight,

sunlight exposure for 20 or 30 minutes,

but rather just looked at hormone levels

throughout the year and found that the increase

in day length correlated with increases

in testosterone and sexual passion.

Now, my opinion, this is a very noteworthy study

because it really illustrates that sunlight

and day length can impact the melatonin pathway

and thereby take the foot off the brake, so to speak,

on testosterone, estrogen, and the desire to mate.

It also emphasizes that sunlight, UVB light,

can directly trigger hormone pathways

and desire to mate and mating behavior.

Now, this study went a step further

in defining the precise mechanism

by which light can impact all these hormones

and this desire to mate.

And here, understanding the mechanism is key

if you want to export a particular protocol

or tool that you might apply.

We talked earlier about how UVB light exposure

to the eyes triggers activation

of these particular neurons within the eye,

and then with centers deeper in the brain

and eventually the pineal gland

to suppress the output of melatonin

and thereby to allow testosterone and estrogen

to exist at higher levels

because melatonin can inhibit testosterone and estrogen.

In this study, they were able to very clearly establish

that it is sunlight exposure to our skin

that is causing these hormone increases

that they observed in mice and humans.

And the way they did that

was to use the so-called knockout technology,

the ability to remove specific genes

within specific tissues of the body.

And what they found is that UVB light,

meaning sunlight exposed skin,

upregulated, meaning increased the activity

of something called P53,

which is involved in the maturation of cells

and various aspects of cellular function.

And the cells they were focused on were the keratinocytes,

which you are now familiar with

from our earlier discussion about the fact

that the epidermis of your skin

contains mainly keratinocytes and melanocytes.

Sunlight exposure increased P53 activity in the skin

and P53 activity was required

for the downstream increases in ovarian size,

in testicular size, in testosterone increases,

in the estrogen increases,

and the various other changes that they observed

at the physiological level

when animals or humans were exposed to sunlight.

So these data are important because what they mean

is that not only is it important

that we get sunlight exposure early in the day

and throughout the day to our eyes,

at least as much as is safely possible,

but that we also need to get UVB sunlight exposure

onto our skin if we want to activate this P53 pathway

in keratinocytes and the testosterone and estrogen increases

that are downstream of that P53 pathway.

So even though the gene knockout studies were done on mice,

they clearly show that if you remove P53 from the skin,

that these effects simply do not occur.

So in terms of thinking about a protocol

to increase testosterone and estrogen mood

and feelings of passion,

the idea is that you would want to get

this two to three exposures per week minimum

of 20 to 30 minutes of sunlight exposure

onto as much of your body

as you can reasonably expose it to.

And when I say reasonably, I mean,

of course you have to obey cultural constraints,

decency constraints,

and of course you have to also obey the fact

that sunlight can burn your skin.

So many people are probably going to ask

what happens if you wear sunscreen?

Well, in theory, because sunscreen has UV protection,

it would block some of these effects.

Now I’m not suggesting that people do away

with sunscreen entirely.

I do hope to do an episode all about sunscreen

in the future because sunscreen is a bit

of a controversial topic.

Skin cancers are a real thing,

and many people are especially prone to skin cancer.

So you need to take that seriously.

Some people are not very prone to skin cancers

and can tolerate much more sun exposure.

You’re probably familiar with the simple fact

that if you’ve gone outside on the beach with friends,

some people get burned very easily, others don’t.

So you really should prioritize the health

and the avoidance of sunburn on your skin.

However, these data and other data point to the fact

that we should all probably be striving

to get more sunlight exposure onto our skin

during the winter months

and still getting sunlight exposure onto our skin

in the summer months,

provided we can do that without damaging our skin.

Another set of very impressive effects of UVB light,

whether or not it comes from sunlight

or from an artificial source,

is the effect of UVB light on our tolerance for pain.

It turns out that our tolerance for pain

varies across the year,

and that our pain tolerance is increased

in longer day conditions.

And as we saw with the effects of UVB

on hormones and mating,

again, this is occurring via UVB exposure to the skin

and UVB exposure to the eyes.

I want to just describe two studies

that really capture the essence of these results.

I’m going to discuss these in kind of a top contour fashion.

I won’t go into it as quite as much depth

as I did the last study,

but I will provide links to these studies as well.

The first study is entitled

Skin Exposure to Ultraviolet B

Rapidly Activates Systemic Neuroendocrine

and Immunosuppressive Responses.

And you might hear that and think,

oh, immunosuppressive, that’s bad.

But basically what they observed

is that even one exposure to UVB light

changed the output of particular hormones

and neurochemicals in the body,

such as corticotropin hormone and beta endorphins,

which are these endogenous opioids.

We’ve all heard of the opioid crisis,

which is people getting addicted to opioids

that they are taking in drug form, pharmaceuticals.

But here I’m referring to endorphins

that our body naturally manufactures and releases

in order to counter pain

and act as a somewhat of a psychological soother also,

because of course, physical pain and emotional pain

are intimately linked in the brain and body.

What they found was that exposure to UVB light

increased the release of these beta endorphins.

It caused essentially the release

of an endogenous painkiller.

Now, a second study that came out very recently,

just this last week, in fact,

published in the journal Neuron,

Cell Press Journal, excellent journal,

is entitled a visual circuit related

to the periaqueductal gray area

for the anti-nosusceptive effects

of bright light treatment.

I’ll translate a little bit of that for you.

The periaqueductal gray is a region of the midbrain

that contains a lot of neurons

that can release endogenous opioids.

Things like beta enkephalin, things like enkephalin,

things like mu opioid.

These are all names of chemicals

that your body can manufacture

that act as endogenous painkillers

and increase your tolerance for pain.

They actually make you feel less pain overall

by shutting down some of the neurons that perceive pain

or by reducing their activity,

not to a dangerous level, right?

They’re not going to block the pain response

so that you burn yourself unnecessarily

or harm yourself unnecessarily,

but they act as a bit of a painkiller from the inside.

If you heard the word anti-nosusceptive,

nociception is basically the perception

or the way in which neurons respond to painful stimuli.

So you can think of nociceptive events

in your nervous system as painful events.

And there I’m using a broad brush.

I realized that the experts in pain will say,

oh, it’s not a really a pain circuit, et cetera, et cetera.

But for sake of today’s discussion,

it’s fair to say that nociception is the perception of pain.

So if this title is a visual circuit

related to the periaqueductal gray,

which is this area that releases these endogenous opioids

for the anti-nosusceptive,

the anti-pain effects of bright light treatment.

The key finding of this study

is that it is light landing on the eyes

and captured by the specific cells

I was talking about earlier,

those intrinsically photosensitive

melanopsin ganglion cells is the long name for them.

But these particular neurons in your eye

and in my eye, incidentally,

that communicate with particular brain areas.

These brain areas have names.

If you want to know them for you aficionados

or for you ultra curious folks,

they have names like the ventral lateral geniculate nucleus

and the intergeniculate leaflet.

The names don’t matter.

The point is that light landing on the eyes

is captured by these melanopsin cells.

They absorb that light,

translate that light into electrical signals

that are handed off to areas of the brain,

such as the ventral geniculate.

And then the ventral geniculate

communicates with this periaqueductal gray area

to evoke the release of these endogenous opioids

that soothe you and lead to less perception of pain.

This is a really important study

because it’s long been known that in longer days

or in bright light environments,

we tolerate emotional and physical pain better.

Previous studies had shown

that it is light landing on our skin

that mediates that effect,

but only in part, it couldn’t explain the entire effect.

This very recent study indicates

that it’s also light arriving at the eyes.

And in this case, again, UVB light,

ultraviolet blue light of the sort that comes from sunlight,

that is triggering these anti-pain

or pain relieving pathways.

So once again, we have two parallel pathways.

This is a theme you’re going to hear

over and over and over again,

not just in this episode,

but in all episodes of the Huberman Lab Podcast,

because this is the way

that your brain and body are built.

Nature rarely relies on one mechanism

in order to create an important phenomenon

and pain relief is an important phenomenon.

So we now have at least two examples

of the potent effects of UVB light exposure

to the skin and to the eyes.

One involving activation of testosterone

and estrogen pathways as it relates to mating,

and another that relates to reducing the total amount

of pain that we experience

in response to any painful stimuli.

So for those of you that are thinking tools and protocols,

if you’re somebody who’s experiencing chronic pain,

provided you can do it safely,

try to get some UVB exposure, ideally from sunlight.

I think the 20 to 30 minute protocol,

two or three times per week is an excellent one.

It seems like a fairly low dose of UVB light exposure.

It’s hard to imagine getting much damage to the skin.

Of course, if you have very sensitive skin,

or if you live in an area of the world

that is very, very bright and has intense sunlight,

particular times of year, you’ll want to be cautious.

Heed the warnings and considerations about sunscreen

that I talked about earlier or about wearing a hat.

But the point is very clear.

Most of us should be getting more UVB exposure from sunlight.

I can already hear the screams within the comments

or the rather the questions within the comments saying,

well, what if I live in a part of the world

where I don’t get much UVB exposure?

And I want to emphasize something that I’ve also emphasized

in the many discussions on this podcast

related to sleep and circadian rhythms and alertness,

which is even on a cloud covered day,

you are going to get far more light energy photons

through cloud cover than you are going to get

from an indoor light source, an artificial light source.

I can’t emphasize this enough.

If you look outside in the morning and you see some sunlight,

if you see some sunlight throughout the day,

you would do yourself a great favor to try and chase

some of that sunlight and get into that sunlight

to expose your eyes and your skin to that sunlight

as much as you safely can.

And when I say as much as you safely can,

never ever look at any light, artificial sunlight

or otherwise that’s so bright that it’s painful to look at.

It’s fine to get that light arriving on your eyes indirectly.

It’s fine to wear eyeglasses or contact lenses.

In fact, if you think about the biology of the eye

and the way that those lenses work,

they will just serve to focus that light

onto the very cells that you want those light beams

to be delivered to.

Whereas sunglasses that are highly reflective

or trying to get your sunlight exposure

through a windshield of a car or through a window

simply won’t work.

I’m sorry to tell you, but most windows are designed

to filter out the UVB light.

And if you’re somebody who’s really keen on blue blockers

and you’re wearing your blue blockers all day,

well, don’t wear them outside.

And in fact, you’re probably doing yourself a disservice

by wearing them in the morning and in the daytime.

There certainly is a place for blue blockers

in the evening and nighttime,

if you’re having issues with falling and staying asleep.

But if you think about it, blue blockers,

what they’re really doing is blocking those short wavelength

UVB wavelengths of light that you so desperately need

to arrive at your retina.

And of course, also onto your skin

in order to get these powerful biological effects

on hormones and on pain reduction.

And in terms of skin exposure,

these data also might make you think a little bit

about whether or not you should wear short sleeves

or long sleeves, whether or not you want to wear shorts

or a skirt or pants.

It’s all going to depend on the context of your life

and the social and other variables

that are important, of course.

I don’t know each and every one of your circumstances,

so I can’t tell you to do X or Y or Z, nor would I,

but you might take into consideration

that it is the total amount of skin exposure

that is going to allow you to capture more or fewer photons,

depending on, for instance,

if you’re completely cloaked in clothing

and you’re just exposed in the hands, neck and face,

such as I am now,

or whether or not you’re outside in shorts and a t-shirt,

you’re going to get very, very different patterns

of biological signaling activation

in those two circumstances.

Many of you, I’m guessing, are wondering

whether or not you should seek out UVB exposure

throughout the entire year or only in the summer months.

And that’s sort of going to depend

on whether or not you experience depression

in the winter months, so-called seasonal affective disorder.

Some people have mild,

some people have severe forms

of seasonal affective disorder.

Some people love the fall and winter and the shorter days.

They love bundling up, they love the leaves,

they love the snow, they love the cold,

and they don’t experience those psychological lows.

So it varies tremendously.

And there are genetic differences

and birthplace origin differences that relate to all this,

but really it has to be considered on a case-by-case basis.

I personally believe, and this was reinforced

by the director of the chronobiology unit

at the National Institutes of Mental Health, Sam Rathar,

that we would all do well to get more UVB exposure

from sunlight throughout the entire year,

provided we aren’t burning our skin

or damaging our eyes in some way.

In addition to that, during the winter months,

if you do experience some drop in energy

or increase in depression or psychological lows,

it can be very beneficial to access a sad lamp,

or if you don’t want to buy a sad lamp,

because oftentimes they can be very expensive,

you might do well to simply get a LED lighting panel.

I’ve described one before,

and I want to emphasize that I have no affiliation whatsoever

to these commercial sources,

but I’ve described one before and I’ll describe it again,

and we can provide a link to a couple examples of these

in the show note captions, excuse me.

This is a 930 to 1000 lux LUX light source

that’s designed for drawing.

It’s literally a drawing box.

It’s a thin panel.

It’s about the size of a laptop.

Very inexpensive compared to the typical sad lamp.

I actually have one,

and I position it on my desk all day long.

I also have enough skylights above my desk.

I’m fairly sensitive to the effects of light.

So in longer days,

I feel much better than I do in shorter days.

I’ve never suffered from full-blown

seasonal affective disorder,

but I keep that light source on throughout the day,

throughout the year.

But I also make it a point to get outside

and get sunlight early in the morning

and several times throughout the day.

And if it’s particularly overcast outside,

or there just doesn’t seem to be a lot of sunlight

coming through those clouds,

I will try to look at that light source

a little bit more each day

in order to trigger these mechanisms.

Now, some people may desire to get UVB exposure

to their skin,

and they want to do that through sources

other than sunlight.

And there, it’s a little bit more complicated.

There are, of course, tanning salons,

which basically are beds of UVB light.

That’s really all they are.

I’ve never been to one.

I know people do frequent them

in certain parts of the world.

There, of course, people are covering their eyes.

They are only getting UVB exposure to their skin typically,

because the UVB exposure, or intensities rather,

tends to be very, very high.

And so you can actually damage your eyes

if you’re looking at a very, very bright

artificial UVB source up close.

So you really have to explore these options for yourself.

Sunlight, of course, being the original

and still the best way to get UVB exposure.

So without knowing your particular circumstances,

finances, genetics, or place of origin, et cetera,

I can’t know whether or not

you need to use artificial sources.

You’re going to have to gauge that.

Meanwhile, getting outside,

looking at and getting some exposure of UVB onto your skin

is going to be beneficial

for the vast majority of people out there.

And in fact, it’s even going to be beneficial

for people that are blind.

People that are blind, provided they still have eyes,

often maintain these melanopsin cells.

So even if you’re low vision or no vision,

getting UVB exposure to your eyes can be very beneficial

for sake of mood, hormone pathways,

pain reduction, and so forth.

A cautionary note, people who have retinitis pigmentosa,

macular degeneration, or glaucoma,

as well as people who are especially prone to skin cancers

should definitely consult with your ophthalmologist

and dermatologist before you start increasing

the total amount of UVB exposure

that you’re getting from any source,

sunlight or otherwise.

There are additional very interesting

and powerful effects of UVB light,

in particular on immune function.

All the organs of our body are inside our skin.

And so information about external conditions,

meaning the environment that we’re in,

need to be communicated to the various organs of our body.

Some of them have more direct access

to what’s going on outside.

So for instance, the cells in your brain

that reside right over the roof of your mouth,

your hypothalamus, they control hormone output

and they control the biological functions

that we call circadian functions,

the one that change every 24 hours.

Well, those are just one or two connections,

meaning synapses away from those cells in your eye

that perceive UVB light, excuse me.

Other organs of your body, such as your spleen,

which is involved in the creation of molecules and cells

that combat infection,

well, those are a long ways away

from those cells in your eye.

And in fact, they’re a long ways away from your skin.

There are beautiful studies showing that

if we get more UVB exposure from sunlight

or from appropriate artificial sources,

that spleen and immune function are enhanced.

And there’s a very logical, well-established circuit

as to how that happens.

Your brain actually connects to your spleen.

Now, it’s not the case that you can simply think,

okay, spleen, turn on, release killer cells,

go out and combat infection.

However, UVB light arriving on the eyes

is known to trigger activation of the neurons

within the so-called sympathetic nervous system.

These neurons are part of the larger thing

that we call the autonomic nervous system,

meaning it’s below or not accessible by conscious control.

It’s the thing that controls your heartbeat,

controls your breathing,

and that also activates or flips on the switch

of your immune system.

When we get a lot of UVB light in our eyes,

or I should say sufficient UVB light in our eyes,

a particular channel, a particular set of connections

within the sympathetic nervous system is activated

and our spleen deploys immune cells and molecules

that scavenge for and combat infection.

So if you’ve noticed that you get fewer colds and flus

and other forms of illness in the summer months,

part of that could be because of the increase in temperature

in your environment,

because typically longer days are associated

with more warmth in your environment

as opposed to winter days, which are short,

when it tends to be colder out.

Well, that’s true, but it’s also the case,

the people around you have fewer colds and flus

and that you will get infected with fewer colds and flus

and other infections,

because if those infections,

whether or not they’re bacterial or viral,

arrive in your body, right?

If you inhale them or they get into your mouth

or on your skin,

your spleen meets those infections with a greater output.

In other words, the soldiers of your immune system,

the chemicals and cell types of your immune system,

that combat infection are in a more ready deployed stance,

if you will.

If you want to know more about the immune system

and immune function,

I did an entire episode about the immune system

and the brain in a,

you can find that at

We talk about cytokines,

we talk about killer cells, B cells, T cells, et cetera,

a lot of detail there.

So we often think about the summer months

and the spring months as fewer infections floating around,

but in fact, there aren’t fewer infections floating around.

We are simply better at combating those infections

and therefore there’s less infection floating around.

So we are still confronted with a lot of infections.

We’re just able to combat them better.

What does this mean in terms of a tool?

What it means is that during the winter months,

we should be especially conscious of accessing UVB light

to enhance our spleen function,

to make sure that our sympathetic nervous system

is activated to a sufficient level

to keep our immune system deploying all those killer T cells

and B cells and cytokines

so that when we encounter the infections,

as we inevitably will, right?

We’re constantly being bombarded with potential infections

that we can combat those infections well.

And as just a brief aside,

but I should mention a brief aside

that’s related to tens of thousands of quality studies.

It is well-known that wound healing is faster

when we are getting sufficient UVB exposure.

Typically that’s associated

with the longer days of spring and summer.

It is known that turnover of hair cells,

the very cells that give rise to hair cells

are called stem cells.

They live in little so-called niches in our skin

with these hair stem cells

and your hair grows faster in longer days.

That too is triggered by UVB exposure,

not just to the skin, but to the eyes.

That’s right.

There was a study published

in the Proceedings of the National Academy of Sciences

a couple of years ago

that showed that the exposure

of those melanopsin ganglion cells in your eyes

is absolutely critical for triggering

the turnover of stem cells in both the skin and hair

and also it turns out in nails.

So if you’ve noticed that your skin, your hair,

and your nails look better and turnover more,

meaning grow faster in longer days,

that is not a coincidence.

That is not just your perception.

In fact, hair grows more, skin turns over more,

meaning it’s going to look more youthful.

You’re going to essentially remove older skin cells

and replace them with new cells

and all the renewing cells and tissues of our body

are going to proliferate,

are going to recreate themselves more

when we’re getting sufficient UVB light to our eyes

and also to our skin.

And so while some of you may think of light therapies

such as red light therapies or UVB therapies

as kind of new agey or just biohacking,

again, a phrase I don’t particularly like,

this notion of biohacking

because it implies using one thing

for a purpose that it was never intended to have.

Well, it turns out that UVB exposure and red light,

as we’ll soon see, is a very potent form

of increasing things like wound healing and skin health

for very logical mechanistically backed reasons.

So while I can’t account for everything

that’s being promoted out there

in terms of this light source

will help your skin look more youthful

or will help heal your scars,

the mechanistic basis for light

having those effects makes total sense.

But what you should consider, however,

is that if the particular light therapy

that you’re considering involves very local application

rather than illuminating broad swaths of skin

and if it has no involvement with the eyes,

meaning there’s no delivery of UVB or red light

or the other light therapy to the eyes,

it’s probably not going to be as potent a treatment

as would a more systemic activation

of larger areas of skin and the eyes.

Now, again, a cautionary note,

I don’t want people taking technologies

that were designed for local application

and beaming those into the eyes.

That could be very, very bad

and damaging to your retinal and other tissues.

Certainly wouldn’t want you taking bright light

of very high intensity of any kind

and getting cavalier about that.

Typically the local illumination of say a wound

or a particular patch of acne

or some other form of skin treatment

involves very high intensity light.

And if the intensity is too high,

you can actually damage that skin.

And so as we’ll talk about in a few moments,

most of those therapies for modifying skin

involve actually burning off a small,

very thin layer at the top of the epidermis

in efforts to trigger the renewal

or the activation of stem cells

that will replenish that with new cells.

So there’s a fine line to be had

between light therapies that are very localized and intense,

which are designed to damage skin

and cause reactivation of new stem cells,

whether or not it’s hair cells or skin cells, et cetera,

versus systemic activation

across broad swaths of skin in the eyes.

You really have to consider this on a case-by-case basis,

but at least for now,

just consider that increases in hormones,

reduction in pain by way of increases in enkephalin

and other endogenous opioids,

improving immune status by activating the spleen

and so on and so on,

really are all the downstream consequence

of illuminating large swaths of skin

and making sure that those neurons within the eye

get their adequate UVB exposure

or other light wavelength exposure,

not simply beaming a particular wavelength of light

at a particular location on the body

and hoping that that particular illumination

at a particular location on the body

is going to somehow change the biology at that location.

Our biology just really doesn’t work that way.

It’s possible, but in general,

systemic effects through broad-scale illumination

and illumination to the eye

combined with local treatments

are very likely to be the ones that have the most success.

Now I’d like to shift our attention

to the effects of light on mood more specifically.

We talked about this

in terms of seasonal affective disorder,

but many of us don’t suffer from seasonal affective disorder.

So I’d like to drill a little deeper

into how light impacts mood.

And here I want to, again, paraphrase the statements

of Dr. Samar Hattar

at the National Institutes of Mental Health.

I should mention the director of the chronobiology unit

at the National Institutes of Mental Health

and perhaps one of the top one to two to three world experts

in how light can impact mood, appetite, circadian rhythms,

and so forth.

Samar stated on the podcast,

and he said in various other venues as well,

that getting as much UVB light in our eyes

and on our skin in the early day

and throughout the day as safely possible

is going to be beneficial for mood.

There’s also another time of day,

or rather I should say a time of night

in which UVB can be leveraged in order to improve mood.

But it’s actually the inverse of everything

we’ve been talking about up until now.

We have a particular neural circuit

that originates with those melanopsin cells in our eye

that bypass all the areas of the brain

associated with circadian clocks.

So everything related to sleep and wakefulness

that’s specifically dedicated to the pathways

involving the release of molecules like dopamine,

the neuromodulator that’s associated with motivation,

with feeling good,

with feeling like there’s possibility in the world,

and so on and so forth.

And other molecules as well, including serotonin

and some of those endogenous opioids

that we talked about before.

That particular pathway involves a brain structure

called the perihabenular nucleus.

The perihabenular nucleus gets input

from the cells in the eye that respond to UVB light,

and frankly, to bright light of other wavelengths as well,

because as you recall, if a light is bright enough,

even if it’s not UV or blue light,

it can activate those cells in the eye.

Those cells in the eye communicate

to the perihabenular nucleus.

And as it turns out, if this pathway is activated

at the wrong time of each 24-hour cycle,

mood gets worse, dopamine output gets worse.

Molecules that are there specifically to make us feel good

actually are reduced in their output.

So while UVB exposure in the morning

and throughout the day is going to be very important

for elevating and maintaining elevated mood,

avoiding UVB light at night is actually a way

in which we can prevent activation

of this eye to perihabenular pathway

that can actually turn on depression.

To be very direct and succinct about this,

avoid exposure to UVB light from artificial sources.

Between the hours of 10 p.m. and 4 a.m.,

and if you’re somebody who suffers from low mood

and overall has a kind of mild depression

or even severe depression, of course,

please see a psychiatrist, see a trained psychologist,

get that treated.

But you would do especially well to avoid UVB exposure

from artificial sources, not just from 10 p.m. to 4 a.m.,

but really be careful about getting too much exposure

to UVB even in the late evening,

so 8 p.m. perhaps to 4 a.m.

I can’t emphasize this enough that if you view UVB light,

you activate those neurons in your eye very potently,

and if those cells communicate

to the perihabenular nucleus, which they do,

you will truncate or reduce the amount of dopamine

that you release.

So if you want to keep your mood elevated,

get a lot of light, UVB light throughout the day,

and at night, really be cautious

about getting UVB exposure from artificial sources.

Now, let’s say you’re somebody who has no issues with mood,

you’re just the happiest person all year long,

or maybe you just have subtle variations in your mood,

you feel great about that.

Turns out that you still want to be very careful

about light exposure between the hours of 10 p.m. or so

and 4 a.m., in fact, even during sleep.

There’s a recent study that just came out

in the Proceedings of the National Academy of Sciences,

and it’s entitled Light Exposure During Sleep

Impairs Cardiometabolic Function.

This is a very interesting study

where they took human subjects, young adults,

and having them sleep in rooms

that had different lighting conditions,

either dim light or slightly bright light.

Now, many people can’t fall asleep in brightly lit rooms,

so they acknowledge this.

These were not very brightly lit rooms.

These were rooms that had just a little bit

of overhead room lighting, 100 lux,

which is not very bright at all,

or they had them sleep in a room that had very dim light,

which is less than three lux.

If you want to get a sense of how bright three lux is

versus 100 lux, I would encourage you

to download the free app Light Meter.

I have no relationship to the app.

It’s a pretty cool app.

However, I’ve used it for a long time

where you can basically point your phone

at a particular light source, sun or otherwise,

and you just press the button,

and it’ll give you an approximate readout of lux,

which is the light intensity that the phone happens

to be staring out at at that location.

It’s not exact, but it’s a pretty good

back of the envelope measure of light intensity.

So these subjects were either sleeping

in a very dim room, three lux is very, very dim,

or a somewhat dim room, 100 lux.

In this study, they measured things like melatonin levels.

They looked at heart rate.

They looked at measures of insulin and glucose management.

Now, in previous episodes, I’ve talked about how glucose,

blood sugar, is regulated by insulin

because you don’t want your glucose levels

to be too high, hyperglycemia, or too low, hypoglycemia,

and the hormone insulin is involved in sequestering

and shuttling glucose in the bloodstream.

Basically, how well you manage glucose in the bloodstream

can be indirectly measured by your insulin levels.

And it’s well-known that sleep deprivation

can disrupt glucose regulation by insulin.

However, in this study, subjects were sleeping

the whole night through.

It just so happens that some of the subjects were sleeping

in this very dimly lit room, three lux,

and other subjects were sleeping

in a somewhat dimly lit room, 100 lux.

What’s incredible about this study

is that both rooms were sufficiently dim

that melatonin levels were not altered in either case.

This is really key.

It’s not as if one group experienced a lot of bright light

through their eyelids and others did not.

Melatonin levels were not disrupted.

And given how potently light can inhibit melatonin,

this speaks to the fact that this very dim condition

of three lux and the somewhat dim condition of 100 lux

was not actually perceived by the subjects,

nor was it disrupting these hormone pathways.

They also looked at glucose responses.

They had people essentially take a fasting glucose test

in different conditions.

I won’t go into all the details,

but here’s what they found.

In healthy adults, even just one night of sleeping

in a moderately lit environment,

this 100 lux environment caused changes,

increases in nighttime heart rate,

which means that the sympathetic nervous system

was overly active as compared to people that slept

in a completely dark or in a very, very dimly lit room,

decreases in heart rate variability.

And here I should point out that heart rate variability

or HRV is a good thing.

We want heart rate variability.

So they saw increases in heart rate,

decreases in heart rate variability,

and increases in next morning insulin resistance,

which is an indication that glucose management

is suffering.

So this is powerful.

The results of this study essentially indicate

that even just one night of sleeping the whole night through

in a dimly lit environment is disrupting the way

that our autonomic nervous system is functioning,

altering so-called autonomic tone,

making us less relaxed is probably the best way

to describe it.

Even though we are asleep,

disrupting the way that our cardiometabolic function

operates such that we have lower heart rate variability

and increased insulin resistance.

This is not a good thing for any of us to experience.

So while we’ve mainly been talking about the positive

effects of UVB light and other forms of light,

now we have two examples,

one from the work of Hattar and colleagues showing

that UVB exposure via the perihepenula

can diminish the output of dopamine and other molecules

that make us feel good.

If that UVB exposure is in the middle of the night

or late evening.

And now we have yet another study performed in this case,

in humans,

indicating that even if we fall asleep and sleep the whole

night through,

if the room that we’re sleeping in has too many locks,

too much light energy,

that light energy is no doubt going through the eyelids,

which it can activating the particular cells in the eye

that trigger an increase in sympathetic nervous system

activation and disrupting our metabolism.

And this study rests on a number of other recent studies

published in Cell,

which is a superb journal and other journals showing that

during the course of a healthy deep night’s sleep,

our body actually transitions through various forms of

metabolic function.

We actually experience ketosis like states.

We experience gluconeogenesis.

We experience different forms of metabolism associated with

different stages of sleep,

not something that we’re going into in depth in this

podcast, we will in a future podcast.

What this study shows is that light exposure,

even in sleep is disrupting our autonomic,

in this case,

the sympathetic arm of the autonomic nervous system in ways

that are disrupting metabolism, probably in sleep,

but certainly outside of sleep.

So that we wake up and have our first meal of the day,

or even if you’re intermittent fasting,

you eat that first meal of the day.

If your sleep is taking place in an environment that’s

overly illuminated,

well, that’s disrupting your cardiac function and your


I’ve been talking a lot about UVB light,

which is short wavelength light.

So UV light, blue light, maybe even some blue green light.

That’s going to be short wavelength light.

Now I’d like to shift our attention to the other end of the

spectrum, meaning the light spectrum,

to talk about red light and infrared light,

which is a long wavelength light.

Many so-called low level light therapies.

The acronym is LLLT,

low level light therapies involve the use of red light and

infrared light.

Sometimes low level light therapies involve the use of UVB,

but more often than not these days,

when we hear LLLT, low level light therapy,

it’s referring to red light and near infrared light


Low level light therapies have been shown to be effective

for a huge number of biological phenomenon and medical


I can’t summarize all of those now.

It would take me many, many hours.

It would be an effective episode for curing insomnia,

but it wouldn’t inform you properly about the use of light

for your health.

Rather, I’d like to just emphasize some of the top contour

of those studies and point out that for instance,

low level light therapy with infrared light has been shown

to be effective for the treatment of acne and other sorts of

skin lesions.

Been some really nice studies actually,

where they use subjects as their own internal control.

So people believe it or not agreed to have half of their face

illuminated with red light or near infrared light.

And the other half of their face serve as a control.

And to do that for several weeks at a time.

And you can see pretty impressive reductions in skin

lesions, reductions in scars from acne and reduction in acne

lesions themselves,

meaning the accumulation of new acne cysts with low level

light therapy using red light and infrared light.

Sometimes, however,

there is a resistance of that acne to the low level light

therapy, such that people will get an initial improvement

and then it will go away despite continuing the treatment.

So you’re probably asking,

or at least you should be asking,

how is it that shining red light on our skin can impact

things like acne and wound healing, et cetera?

Well, to understand that we have to think back to the

beginning of the episode where I described how long

wavelength light, such as red light and near infrared light,

which is even longer than red light can pass through certain

surfaces, including our skin.

So our skin has an epidermis, which is on the outside.

And the dermis, which is in the deeper layers,

red light and infrared light can pass down into the deeper

layers of our skin where it can change the metabolic

function of particular cells.

So let’s just take acne as an example, within the dermis,

the deep layers of our skin,

we have what are called sebaceous glands that actually make

the oil that is present in our skin.

Those sebaceous glands are often nearby hair follicles.

So if you’ve ever had an infected hair follicle,

that’s not a coincidence that hair follicles tend to get


Part of it is because there’s actually a portal down and

around the hair follicle,

but the sebaceous gland is where the oil is created.

That is going to give rise to, for instance, acne lesions.

Also in the dermis and the deep layers of the skin are the

melanocytes. They’re not just in the epidermis.

They’re also in the deeper layers of the skin.

And you have the stem cells that give rise to additional

skin cells.

If the top layers of the epidermis are damaged,

those stem cells can become activated.

And you also have the stem cells that give rise to hair


So by shining red light or near infrared light on a

localized patch of skin,

provided that red light is not of such high intensity that

it burns the skin,

but is of sufficient intensity that provides just a little

bit of damage to the upper layers of the skin, the


and that it triggers certain biological pathways within the

cells of the sebaceous gland and the stem cells within the

hair cell niche and the stem cells and skin.

What happens is the top layers of the skin are basically

burned off by a very low level of burn and,

or the cells in the deeper layer start to churn out new

cells, which go and rescue the lesion,

essentially clear out the lesion and replace that lesion

with healthy skin cells.

This does work in the context of wound healing,

getting scars to disappear.

It also works to remove certain patches of pigmentation.

There are sometimes cases where people will get a red

blotchiness due to certain skin conditions or some darker

pigmentation that they want removed or that they need

removed because it’s a potential skin cancer threat.


how is red light actually doing it within the cells of the

sebaceous gland, the stem cells, et cetera?


long wavelength light can actually get deep into the skin.

I mentioned that before,

but can also get into individual cells and can access the

so-called organelles,

which I described at the beginning of the episode,

in particular,

they can access the mitochondria,

which are responsible for producing ATP.


the simple way to think about this for sake of this

discussion is that as cells age,

and in particular in very metabolically active cells,

they accumulate what are called ROSs,

reactive oxygen species.

And as reactive oxygen species go up ATP,

energy production in those cells tends to go down.

It’s a general statement,

but it’s a general statement that in most cases is true.

There are some minor exceptions that don’t concern us that

have to do with cell types different than the ones that I’m

talking about now.

So the way to think about this is that red light passes into

the deeper layers of the skin,

activates mitochondria,

which increases ATP and directly or indirectly reduces these

reactive oxygen species.

These reactive oxygen species are not good.

We don’t want them.

They cause cellar damage, cellar death,

and for the most part,

just inhibit the way that our cells work.

So if you’ve heard of red light or near infrared light

therapies designed to heal skin or improve skin quality or

remove lesions or get rid of scars or unwanted pigmentation,

that is not pseudoscience.

That is not woo science.

That is grounded in the very biology of how light interacts

with mitochondria and reactive oxygen species.

Some of you may also find it interesting to note that some

of the cream-based treatments for acne, for instance,

like retinoic acid,

retin A is actually a derivative of vitamin A and the

pathway involving retinoic acid and vitamin A believe it or

not is very similar to the natural biological pathway by

which photo pigments in the eye convert light information in

to biological changes within those cells.

So the key point here is that light is activating particular

pathways in cells that can either drive death of cells or

can make those cells essentially younger by increasing ATP by

way of improving mitochondrial function.

And in recent years,

there’ve been some just beautiful examples that exist not

only in the realm of skin biology,

but in the realm of neurobiology whereby red light and near

infrared light can actually be used to enhance the function

of the cells that for instance,

allow us to see better and indeed cells that allow us to

think better.

So now I’d like to review those data because not only are

they interesting in their own right,

but they also point to some very interesting and powerful

application of low cost or zero cost tools that we can use

to improve our vision.

If you are somebody who’s interested in the use of red

light or near infrared light,

so-called LLT low-level light therapies for treatment of

dermatologic issues.

So anything related to skin,

I will include a link to a excellent set of reviews.

The first one is light emitting diodes in dermatology,

a systematic review of randomized controlled trials.

That one includes review of a very large number of studies

came out just a few years ago in 2018.

And I think it’s very clearly and cleanly laid out for

anyone to access.

You can see the degree of effects of red light,

for instance, on treatment of acne or scarring, et cetera.

And I’ll also provide a link to another review,

which is low-level light therapy in skin,

stimulating healing and restoring.

So for those of you that are interested again,

in dermatologic issues and the kind of restoring

youthfulness and the kind of general themes of,

of anti-aging and longevity and how red light therapies can

be used for that.

I will encourage you to take a look at those reviews.

What you’re going to find is that rarely, if ever,

is there a study looking at whole body red light

illumination for sake of treating and improving skin?

And I mentioned this because I get a lot of questions about

infrared sauna and global illumination with red lights.

We’ll talk more about cases where global illumination of

your whole body or your whole face with red lights might be

useful, but in terms of infrared sauna,

I’ve mentioned on this podcast before,

and I will certainly go deeper on this in an upcoming


all about the use of heat and temperature for augmenting

our biology.

But in general,

infrared saunas don’t get hot enough temperature wise in

order to trigger some of the important effects on growth

hormone and heat shock proteins.

And some of the other things that sauna has been shown to be

excellent for that’s a general statement.

I realized there are some infrared saunas that do get hot


There are very few data on the use of whole body

illumination with infrared saunas.

They really point to any specific mechanistically supported


almost all the positive effects that you’re going to see of

red light and low level light therapies.

Certainly the ones discussed in the reviews that I just

mentioned are going to be the consequence of very directed

illumination of particular patches of skin that are seeking

repair that people are seeking the repair of.

So again,

I don’t want to disparage infrared saunas,

but in general,

they don’t get hot enough to trigger most of the positive

effects that sauna have been demonstrated to have.

And it’s unclear at all as to whether or not they can

enhance skin quality,



you know,

top layers of skin that are damaged,

repair acne,

et cetera.

So more on heat saunas and infrared saunas in their

comparison in an upcoming episode.

So let’s talk about a clear set of examples where red light

and near infrared light have been shown to have positive

effects on our health.

And these are the data that I referred to at the beginning

of the episode from Dr.

Glenn Jeffrey at university college, London,

who again is a longstanding member of the neuroscience

community working on visual neuroscience and who over the

last decade or so has really emphasized the exploration of

red light and near infrared light for restoration of

neuronal function.

As we age,

this is absolutely critical.

We know that we don’t accumulate many new brain cells as we

get older.

And in some areas of our nervous system,

such as our neural retina,

which is the part of our eye that’s responsible for

translating light information to electrical signals so that

we can see we don’t get any new cells after the time in

which we were born.

So the ability to keep our neurons healthy is extremely

important for our visual system,

extremely important for our hippocampus,

an area of the brain involved in memory.

And should just mention that even if people don’t get


there’s always going to be some degree of age-related


Sadly, nobody is as cognitively sharp in the years before

they die as they are 20 years before that it’s just never

the case.

We’re all getting worse at thinking, feeling, perceiving,

et cetera.

The question is how quickly we are getting worse.

So any mechanism by which we can preserve or reverse

neuronal function turns out to be immensely beneficial.

The Jeffrey lab has published two studies in recent years on

humans that looked directly, no pun intended,

at how red light and near infrared light can improve visual


I’m going to describe the parameters of those studies.

And then I’m going to describe what they found exactly.

The mechanistic motivation for these studies, again,

traces back to this effect of light on mitochondria.

So to go a little bit deeper into that mechanism,

just briefly so that you can frame any potential protocol

that you would develop.

When light arrives on cells, including neurons,

that light can penetrate into the cells.

If it’s of the appropriate wavelength,

red light can do that.

It can get into cells.

It can access the mitochondria.

It can increase ATP in general.

Anytime ATP is doing its thing to increase energy in cells,

it’s involving this thing called cytochrome C,

which is an oxidase.

Anytime you hear ACE, A-S-E in biology,

it’s going to be an enzyme.

It’s involved in some process of degrading a molecule and

creating another molecule typically.

So ATP and cytochrome C is going to give you ATP.

Now that’s a great thing,

but it creates a by-product.

It breaks things down such that you get these ROSs,

these reactive oxygen species.

And those reactive oxygen species,

for those of you that want to know,

are involved in things like redox signaling and reactive

oxygen species actually change which genes are made in a


So the goal of any treatment to keep neurons or other cells

youthful and functioning well and to prevent or reverse

aging is going to be to increase ATP and to reduce reactive

oxygen species.

And in doing so to disrupt some of the normal pathways

associated with aging.

The Jeffrey lab approached these studies with that

understanding of how mitochondria and reactive oxygen

species and ATP work.

And what they did was exquisitely simple to the point of

being elegant.

And what they found was really, really exciting.

What they did is they had people,

subjects that were either younger.

So in their twenties or 40 years old or older,

view red light of about 670 nanometers,

670 nanometers would appear red to you and me.

They had, they had them do that, excuse me,

at a distance that was safe for their eyes.

So at about a foot away,

now a foot away from a very intense red light could actually

be damaging to the eyes.

So they had them do this at about a foot away from a red

light that was of low enough intensity that did not damage

the eyes.

And they had them do that anywhere from two to three minutes

per day.

And in one study,

they had them do that for a long period of time of about 12


And in the other study,

they had them do that just for a couple of weeks.

What’s remarkable is that when you collapse the results

across these two studies,

what they found is that when looking at these subjects

ranging from 28 years old to about 72 years old,

the major findings were that in individuals,

40 years old or older.

So in the 40 to 72 year old bracket,

but not in the subjects younger than 40 years old,

they saw an improvement in visual function.

That improvement in visual function was an improvement in

visual acuity,

meaning the ability to resolve fine detail and using a

particular measure of visual function,

which is called the Triton exam, T-R-I-T-A-N,

Triton exam,

which specifically addresses the function of the so-called

short wavelength cones,

the ones that respond to green and blue light.

They saw a 22% improvement in visual acuity,

which in the landscape of visual testing is an extremely

exciting result.


So I think in most studies of improvements of vision,

you’d be very excited to see an improvement of 5% or 10%.

So a 22% improvement in visual acuity,

even though it’s in this very specific form of visual

testing, this Triton exam or this Triton score.


that turns out to be very significant and translates to the

real world in an important way.

In particular,

as we age,

we tend to lose certain neurons within our retina,

but we don’t tend to lose cones.

We tend to lose rods.

We tend to lose other cells within the retina,

including the cells that connect the eye to the brain,

the so-called ganglion cells.

Cones for whatever reason are pretty resilient to age

related loss.


because rods and cones,

both are not just among the most metabolically active cells

in your entire body,

but the most metabolically active cells in your entire body.

That’s right.

Your rods and cones are the cells that demand,

and that use the most energy of all the cells in your body,

not your skin cells,

not your spleen cells,

not your stomach cells.

Even if you talk a lot,

not the cells that are responsible for moving your mouth.

It is the rods and cones of your neural retina that are

responsible for using the most amount of ATP and energy in

your entire body.

And because of that,

those cells tend to accumulate a lot of reactive oxygen

species as we age.

Red light of the sort used in these studies was able to

reduce the amount of reactive oxygen species in the rods and

cones and to rescue the function of this particular cone

type, the short wavelength and medium wavelength cones,

which if you think about the study is a little bit

surprising because it was red light and near infrared light,

not short wavelength light that was used in order to create

this improvement in cellular function.

But if you step back a little bit further,

it makes perfect sense because there’s nothing specific

about the red light in the sense that it’s not that it gets

delivered only to red cones,

that red light and near infrared light is being absorbed by

all the photoreceptors within the eye,

the rods and the blue cones and the green cones and the red

cones. It’s just that the red cones absorb that light best.

So the important takeaway here is that viewing red light and

near infrared light at a distance at which it is safe for

just a couple of minutes each day allowed a reversal of the

aging process of these neurons,

which some people have heard me say before,

and I’ll just say it again,

the retina, including your photoreceptors,

are not just connected to your brain.

They’re not just near your brain.

They are actual central nervous system tissue.

They are the only two pieces of your brain.

Many of your neural retinas are the only two pieces of your

brain that reside outside your skull,

or at least outside the cranial vault.

So here we’re seeing a reversal of the aging process in

neurons by shining red light on those neurons.

Now, of course,

the Jeffrey lab is primarily interested in vision and

humans are most dependent on vision as a sense to navigate

the world and survive. So this is really wonderful here.

We’re looking at a therapy that can reverse age-related

vision loss, at least in some individuals.

But as you can imagine,

the study was also done on these cells because they reside

outside the skull and you can shine light directly on them.


I’m sure that there are many people out there who are

interested in how they can improve the function,

say of the neurons in their brain responsible for memory.

And in a few minutes,

I’ll describe the non-invasive applications of light to try

and restore the function of those cells as well.

So a little bit more about the studies from the Jeffrey lab.

One of the things that they observed was a reduction in so

called drusen, D-R-U-S-E-N.

Drusen are little fatty deposits,

little cholesterol deposits that accumulate in the eye.

As we age,

we’ve all heard about cholesterol within our veins and

arteries and how that can clog our veins and arteries and

how of course,

clogging of veins and arteries is not a good thing.


our neural retina being so metabolically active requires a

lot of blood flow.

It’s heavily vascularized and drusen are a special form of

cholesterol that accumulate in the eye.

As it turns out these red light and near infrared light

therapies explored by the Jeffrey lab,

were able to actually reduce or reverse some of the

accumulation of drusen.

And so in addition to reducing reactive oxygen species,

the idea in mind now is that red light may actually reduce

cholesterol deposits and reactive oxygen species in order to

improve neuronal function.

So what should you and I do with these results or should we

do anything with these results?

Well, first of all,

I want to emphasize that even though these studies are very

exciting, they are fairly recent.

And so more data as always are needed.

There’s some additional features of these studies that I

think are also important to consider.

First of all,

the exposure to red light needed to happen early in the day,

at least within the first three hours of waking.

How would one do that?


nowadays there are a number of different red light panels and

different red light sources that certainly fall within the

range of red light and near infrared light that one could


I don’t have any affiliation to any companies or products

that promote or make those red light therapies.

I do own a red light panel.

So I confess I have started using this protocol.

I am older than 40 years old.

I also have been experimenting with these red light panels as

a way of addressing other changes in biological tissues for

which I’m doing blood work, et cetera.

And I’m going to talk about that in a future episode.

But that of course is what I call anecho data.

It only relates to my experience.

So today,

and certainly on all episodes of the human lab podcast,

we emphasize peer reviewed studies,

almost exclusively talking about anecho data only when

highlighting it as anecho data.

So if you’re somebody who wants to explore red light

therapy, here’s what you need to do.

You need to make sure that that red light source,

whatever source you happen to use,

whether or not you purchase it or make one.

And in fact,

these red light sources are very, very easy to make.

You could essentially take a bright flashlight and cover it

with a film or a filter that would only allow particular

long wavelengths to pass through.

This would be very easy to look up online and figure out

how to do this.

You could probably do this for, you know,

just a few dollars,

or you could purchase a red light unit.

If that was within your budget and something that you’re

interested in,

you want to make sure that it’s not so bright that you’re

damaging your eye.

A good rule of thumb is that something isn’t painful to look

at. And in fact,

I should just emphasize that anytime you look at any light

source, sunlight, or otherwise that’s painful,

it makes you want to squint or close your eyes.

That means it’s too bright to look at without closing your

eyes. Okay. That’s sort of a duh,

but I would loathe to think that anyone would harm

themselves with bright light in any way.

I don’t just say that to protect us.

I say that to protect you, of course,

because you are responsible for your health.

And again, retinal neurons do not regenerate.

Once they are gone and dead, they do not come back.

There’s no technology to replace them at this current state

in time.

So please don’t damage your retinas.

So is a red light source safe to look at if it is not

painful to look at?

Chances are it is.

And yet I would still encourage you to talk to your

optometrist or ophthalmologist before getting into any

extensive protocols.

But if you are still determined to pursue the sorts of

protocols that are in the Jeffrey studies,

certainly we’ll provide a link to those studies.

Again, it involves looking at these red light panels,

blinking aloud for two minutes to three minutes,

every morning for a period of two weeks or more.

And if you’re older than 40,

that could very well have an effect.

If you’re younger than 40, excuse me,

that’s unlikely to have an effect.

At least that was what was observed in these particular


The lights were not flashing.

It was continuous illumination.

Again, you’re allowed to blink.

It does not have to even be direct illumination.

It can be somewhat indirect illumination,

much as we described for the use of UVB light before.

The wavelength of light is important.

It is red light and near infrared light that is going to be

effective in this scenario.

The authors of this study emphasized that it was red light

of 670 nanometers in wavelength and near infrared light of

790 nanometers in wavelength that were effective and that

those wavelengths could be complimentary.

That’s probably why, or maybe it’s just coincidental,

but it’s a fortunate coincidence that a lot of the

commercially available red light panels that you’ll find

out there combine both red light and near infrared light.

However, I want to emphasize that most of the panels that

are commercially available are going to be too bright to

safely look at very close up.

And in fact,

that’s why most of those red light panels are designed for

illumination of the skin and oftentimes arrive in their

packaging with eye protectors that are actually designed to

shield out all the red light.

So take the potential dangers of excessive illumination of

the eyes with any wavelength of light seriously.

But if you’re going to explore 670 and 790 nanometer light

for sake of enhancing neuronal function,

set it at a distance that’s comfortable to look at.

And that doesn’t force you to squint or doesn’t make you

feel uncomfortable physically,

as if you need to turn away during the period of that two to

three minute illumination each day.

In terms of turning away from light,

I’ll just briefly mention that that is not an accident or a

coincidence that you have that response to very bright light.

There is a so-called photic avoidance pathway that involves

cells within your retina,

these ganglion cells that communicate with yet another brain

station, a certain area of your thalamus that communicate to

areas of your brain that are associated with pain.

So literally that can trigger headache and that can trigger

the squint reflex.

Biology is just beautiful in this way.

Too much light is bad for us in that it can damage our eyes

and other aspects of our body.

So if we look at a light, that’s too bright,

our eyes send a signal to the brain that gives us a sort of a

headache and a desire to squint and turn away.

So that can be a useful guide in terms of gauging how bright

a light should be,

or at least how far away you should be from a bright source

in order to safely engage with that light source.

So the studies I just described, once again,

involve the use of red light early in the day,

within three hours of waking and are for the sake of

improving neuronal function.

Red light has also been shown to be beneficial late in the

day and even in the middle of the night.

And when I say middle of the night,

I’m referring to studies that explore the use of red light

for shift workers.

I know that most people are not working in the middle of the

night, at least I hope they’re not,

but some of you may do that from time to time.

All-nighters for studying, I confess,

I still pull all-nighters every once in a while to prepare

things like podcasts and other deadlines really try not to

happens less and less as I get older,

because I think I get more disciplined and,

or less good at pulling all-nighters.

But I realized that many people are doing shift work where

they have to work certainly past 10 PM,

or maybe they’re taking care of young children in the

middle of the night and they have to be up.

In that case, red light can actually be very beneficial.

And nowadays there are a lot of sources of red light

available just as red light bulbs.

You don’t need a panel.

So what I’m basically saying is that it can be beneficial to

use red lights at night.

The study I’d like to emphasize in this context is entitled

red light,

a novel non-pharmacological intervention to promote alertness

in shift workers.

So beautiful study,

they explored the use of different wavelengths of light.

So blue light of 460 nanometers or red light or dim white

light of different brightnesses, et cetera.

And looked at things like melatonin,

how much does light of a given color and intensity suppress


They looked at cortisol, a stress hormone.

They looked at wakefulness,

how much or to what degree could a given color of light

increase wakefulness at different hours of the day.

The takeaway from this study is very clear.

If you need to be awake late at night for sake of shift work

or studying or taking care of children, et cetera,

red light is going to be your best choice because if the red

light is sufficiently dim,

it’s not going to inhibit melatonin production.

And it’s not going to increase cortisol at night.

Cortisol should be high early in the day,

or at least should be elevated relative to other times a

day. If you are healthy,

a late shifted increase in cortisol, however,

9 p.m. cortisol,

10 p.m. cortisol is well-known to be associated with

depression and other aspects of mental health rushes a

mental illness.

So if you do need to be awake at night or even all night,

red light is going to be the preferred light source.

And in terms of how bright to make it well,

as dim as you can,

while still being able to perform the activities that you

need to perform, that’s going to be your best guide.

I’ll provide a link to this study as well.

Again, it’s a really important study because it emphasized

that there are forms of light,

red light provided it’s dim that can allow you to stimulate

the alertness that light landing on the eyes can provide.

So it allows you to stay awake and to do whatever work that

you need to do.

It does not seem to alter melatonin production.

So that’s good.

It does not seem to alter levels or timing of cortisol


So yet another case where red light used correctly can be


Up until now,

we’ve been talking about the effects of shining different

wavelengths of light on the skin or on our eyes and the

downstream health consequences of that illumination.


one of the most important goals of science and medicine is to

figure out how to change the health of our brain.

And of course our brain is contained within our skull and

therefore we can’t just shine light onto the outside of our

head and expected to change the activity of neurons deep

within the brain,

unless those neurons are linked up with our eyes or with our


Because it turns out,

even though there are a lot of brain areas that are

connected through neural circuits and hormone circuits to

our eye and believe it or not also to our skin,

many brain areas are not brain areas,

such as the hippocampus,

which is involved in learning and memory brain areas,

such as our neocortex.


some areas of our neocortex,

such as our visual cortex are indirectly linked to our eyes.

So if we shine light in our eyes,

we can change the activity of neurons in our neocortex,

but there are other brain areas that are not directly or

even indirectly connected to our visual system,

not at least in any immediate way.

So that raises the question of how do you change the

activity of neurons in the brain?

Well, there’s pharmacology.

You can take pills,

you can inject drugs that will change the pharmacology of

neurons and the way they operate in fire.

Of course,

antidepressants are one such instance.

Opioid drugs are another,

but there’s a huge array of psychoactive compounds,

meaning compounds that will change the levels of chemicals

in your brain.

Some of those work.

Many of them also carry side effects.

It’s all rather indirect,

meaning you have lots of different cells in different areas

of your brain that utilize the same chemicals.

So a drug, for instance,

to increase serotonin for sake of improving depression will

also often have the effect of reducing certain neurons

output of serotonin in the hippocampus and cause changes

in appetite or changes in libido and so on and so forth.

You could imagine using electrical stimulation,

putting wires into the brain and stimulating specific brain

areas in order to activate the neurons in those brain areas.

And certainly that works and has been done experimentally

and is done during neurosurgery exams, et cetera,

but involves removing a piece of skull.

So that’s not very practical.

In principle,

light would be a wonderful way to modulate the activity of

neurons deep within the brain.

But again, the skull is in the way.

Recent studies, however,

have figured out ways that light can be delivered to the

eyes to change global patterns of firing in the brain in

ways that can be beneficial to the brain.

And the work that I’m referring to now is mainly the work of

Li-Wei Tsai at MIT,

Massachusetts Institute of Technology and her colleagues.

And what they’ve discovered is that there’s a particular

pattern of brain activity called gamma activity.

Gamma activity is one so-called wavelength of electrical

activity in the brain.

So not wavelengths of light,

but wavelengths of electrical activity in the brain that can

be restorative for certain aspects of learning and memory and

can actually help create molecular changes in neurons that

lead to clearance of debris and even reductions in age

related cognitive decline.

So the way to think about brain waves and brain oscillations

is that neurons are electrically active that involves

chemicals, et cetera,

and they can be active in very slow, big wave forms.

So you can think of, you know, delta waves, meaning,

so you can imagine a wave of electrical activity that comes

along very infrequently.

So a given neuron fires and then some period of time later

fires, and then some period of time, even later fires,

or you can imagine that that same cell is very active fires,

fires, fires, fires, fires.

You can imagine if it’s firing very often,

it’s going to be short wavelength, right?

Shorter gaps between firing, or if it’s firing very seldom,

you’re going to think about that as longer wavelength

firing turns out that gamma waves are one pattern of firing

that lead to downstream metabolic functions and biological

functions that end up clearing away debris that’s associated

with aging in cells.

And that also lead to molecular changes that enhance the kind

of youthfulness of neurons, so to speak.

How do we induce gamma oscillations within the brain?

Well, what Liwei, Tsai and colleagues have beautifully shown

is that by delivering certain patterns of light flicker,

so lights going on and off at a particular frequency,

the brain as a whole starts to entrain,

meaning it matches to those particular patterns of light

flicker, even though many of the brain areas that do this

are not directly within the visual system

or visual pathway.

So the studies that I’m referred to are several,

but the one that I’d like to highlight is entitled

gamma entrainment binds higher order brain regions

and offers neuro protection.

What they essentially did was to expose subjects to 40 Hertz,

which is a particular frequency of illumination to the eyes.

So it’s light goes on, light goes off, light goes on,

light goes off at a frequency of 40 Hertz.

And when they did that and they recorded the activity of

neurons within the brain,

not just within the visual areas of the brain,

but within other areas as well,

they observed increased gamma oscillations,

meaning that the electrical activity of the brain at large

started to match to the patterns of light that were

delivered to the eyes.

This is really exciting and very unique from the different

types of phototherapies that we’ve been talking about up

until now,

all the patterns of phototherapy that we’ve been talking

about up until now involved constant illumination with a

given wavelength here,

it is wavelength generating patterns of illumination light

on light off light on light off at a particular frequency.

So what they found for instance,

using this pattern of stimulation, and by the way,

the stimulation was called Janice gamma entrainment using

sensory stimulation.

So G E N U S gamma entrainment using sensory stimulation had

a number of really interesting effects.

First of all,

it reduced so-called amyloid plaques and phosphorylated tau

amyloid plaques and phosphorylated tau are associated with

Alzheimer’s and normal age-related cognitive decline.

So this is incredible, right?

A pattern of flashing light delivered to the eyes creates a

pattern of neuronal firing,

not just in the visual areas of the brain,

but in other areas of the brain as well,

that in turn trigger molecular pathways that reduce some of

the markers and the cause of age-related cognitive decline

and Alzheimer’s.

And in parallel to that,

they observed an up-regulation of some of the biological

pathways that lead to enhancement of neuronal function,

maintenance of synapses,

which are the connections between neurons and so on and so


They have discovered and list out a huge number of these

biological effects,

both the reduction in bad things, so to speak,

and the improvement in good biological pathways.

And I find these studies so exciting because first of all,

they’re non-invasive, right?

There’s no drilling through the skull.

They are very tractable and in the experimental sense,

meaning that you could imagine that if 40 Hertz stimulation

turns out to be the very best stimulation protocol to induce

these gamma oscillations, well, great,

but because it’s non-invasive,

it’s fairly easy to explore 50 Hertz stimulation,

a hundred Hertz stimulation, 20 Hertz stimulation,

and to do that with different wavelengths of light.

And so that’s, what’s happening now,

the PsyLab and other labs are really starting to explore the

full range of variables that can impact oscillations within

the brain and their downstream consequences.

So again, this is phototherapy,

but phototherapy of a very different sort that we’ve been

talking about up until now.

It’s phototherapy designed to trigger activation of

biological pathways far away from the very tissue that’s

being illuminated.

And it calls to mind the same sorts of mechanisms that we

were talking about earlier,

where illumination of the skin with UVB light is setting off

an enormous number of different cascades and different

organs and tissues, including the spleen, the testes,

the ovaries and so on.

So again, light has these powerful effects,

both locally on the cells that the light is delivered to,

but also systemically in terms of the cells that are

changing their electrical and chemical outputs are modifying

lots and lots of biological programs.

Is there an actionable tool related to these studies yet?

Well, that sort of depends on how adventurous you are right


These studies are being explored in the context of clinical

trials in people with Alzheimer’s dementia and other forms

of neuronal degeneration.

Is it dangerous to look at a 40 Hertz flickering light?

Well, in general, the answer is going to be no.

However, if you’re prone to epilepsy, for instance,

staring at a flickering light of a given continuous

frequency can induce seizure, right?

That might surprise some of you, but it shouldn’t,

because as this study illustrates,

and as anyone who’s ever been out at night to a club or

something illustrates, when you look at a strobe light,

for instance, your whole world of visual perception changes,

but actually the rhythm at which you perceive music,

at which you perceive conversation,

at which you perceive the movement of your body actually

changes according to the patterns of visual flicker.

In most cases, strobe,

if we’re using the sort of club dancing example,

your brain is in training to its outside environment.

So given the power of flickering lights to entrain brain

rhythms, I think at this stage,

it’s probably too preliminary to really suggest a specific


but I would definitely keep an eye out for these sorts of


They are coming out all the time.

And I think in a very short period,

we’re going to see specific protocols that one could

potentially use even at home.

And of course these are non-invasive protocols in order to

place the brain into a particular state,

not just for sake of offsetting neurodegeneration,

but also for enhancing focus,

for enhancing the transition into sleep and other brain

states as well.

Today I covered what I would say is a lot of information.

My goal was to give you an understanding of how light can

be used to change the activities of cells organelles within

those cells, entire organs,

and how that can happen locally and systemically.

We talked about the power of light to impact our biology at

the endocrine level, neuronal level, immune level, mood,

et cetera,

through both illumination of the eyes and the skin and other

tissues as well.

I realized that even though this was a lot of information,

there are many aspects of phototherapy that I did not cover.

I know there’s a lot of interest nowadays, for instance,

in the use of red light and other wavelength light therapies

for ovarian health and testicular health.

In fact, I get a lot of questions such as,

can red light be used to improve testosterone output?

And if so,

is that best accomplished by shining red light on the skin

or directly on the gonads, on the testicles?

I’m going to cover those data at a future time.

Right now, the studies that have been done in rodents,

I don’t think are easily enough translated to humans.

And the studies that are happening in humans now are

exciting in the sense that they hold a lot of potential,

but the data aren’t clear yet.

However, the data using UVB on the skin of men and women in

order to increase hormone,

in particular testosterone and estrogen output,

those data I think are very exciting and very actionable.

We talked about those earlier.

So if you want more information on how phototherapy can be

used, certainly we will do another episode on phototherapy

in these other contexts.

If you’re learning from and or enjoying this podcast,

please subscribe to our YouTube channel.

That’s a terrific zero cost way to support us.

In addition,

please subscribe to the podcast on Apple and Spotify.

And on Apple,

you have the opportunity to leave us up to a five-star


If you have questions or feedback or comments or suggestions

about topics that you’d like us to cover in a future podcast


or guests that you would like me to interview for the

Huberman Lab podcast,

please put all those suggestions,

comments and questions in the comment section on YouTube.

We do read all the comments.

In addition,

please check out the sponsors that we mentioned at the

beginning of the episode.

That’s the best way to support this podcast.

And as mentioned at the beginning of today’s episode,

we are now partnered with Momentous Supplements because

they make single ingredient formulations that are the

absolute highest quality and they ship international.

If you go to slash Huberman,

you will find many of the supplements that have been

discussed on various episodes of the Huberman Lab podcast.

And you will find various protocols related to those


If you’re not already following us on Instagram and Twitter,

please do so.

It’s Huberman Lab on Instagram.

It’s also Huberman Lab on Twitter.

And at both places,

I provide science and science-based tools.

Some of which overlap with the content of this podcast,

much of which is unique from the content of this podcast.

If you’re not already subscribing to our newsletter,

you might consider doing so.

It’s the so-called neural network newsletter.

You can find it at

Just go into the menu,

look up neural network newsletter.

You provide us your email.

It is zero cost.

We provide summaries of podcasts,

summaries of actionable protocols and so forth.

We do not share your email with anybody else.

And we have a very clear privacy policy there at that

website if you choose to explore it.

So thank you once again for joining me today

for this deep dive discussion into phototherapies,

meaning the power of light to modulate our biology

and health.

And as always,

thank you for your interest in science.


comments powered by Disqus