Huberman Lab - Dr. Samer Hattar: Timing Your Light, Food, & Exercise for Optimal Sleep, Energy & Mood

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, I have the pleasure of introducing Dr. Samer Hattar

as my guest on the Huberman Lab Podcast.

Dr. Hattar is the chief of the section

on light and circadian rhythms

at the National Institute of Mental Health

in Bethesda, Maryland.

Dr. Hattar has many important discoveries to his name.

He was one of a handful of groups

that discovered the light-sensing neurons in the eye

that set the circadian clock.

This was a fundamental discovery made in the early 2000s

that has led to an enormous number of additional discoveries

on how light regulates our sleep, our immune system,

our mood, mental health, metabolism, feeding,

and many other important processes.

If ever there was somebody who understands

how all of these processes interact

and can inform best practices for our daily behaviors,

it’s Dr. Hattar.

During our discussion today,

Dr. Hattar answers questions

that are absolutely essential for us to know

about our health and wellbeing.

For instance, how to align our sleep schedule

with our activity schedule, such as exercise,

and how to align light, activity, and exercise

with our feeding rhythms.

He presents a new model of how light, activity,

and feeding rhythms converge to support optimal health,

and when those are not aligned correctly,

how our mental and physical health can suffer.

It’s a discussion that is rich with scientific mechanism,

made clearly, of course, so everybody can understand,

as well as specific protocols to deal with shifts

in day length, shifts in activity,

and in order to optimize sleep, metabolism,

and wellbeing of various kinds.

I learned so much from Samer, as I always do.

He is an absolute wealth of knowledge

on all things related to light and circadian rhythms,

physiology, and neuroscience.

I don’t think you’ll find anyone else as knowledgeable

about these topics as Samer,

and so I’m delighted that he joined us here on the podcast

to share this information.

Before we begin, I’d like to emphasize

that this podcast is separate

from my teaching and research roles at Stanford.

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And now my conversation with Dr. Samer Hattar.

Samer, thanks for sitting down with me.

My pleasure.

Yeah, we go way back.

So you are best known in scientific circles

for your work on how light impacts mood,

learning, feeding, hunger, sleep, and these sorts of topics.

So just to kick the ball out onto the field, so to speak,

how does light impact the way we feel?

So when I get up in the morning,

I have the opportunity to interact with light

in certain ways or to avoid light in certain ways.

I have the opportunity to interact with sunlight

or with artificial light.

Maybe you could just wade us into what the relationship is

between light and these things like mood and hunger,

et cetera.


So, I mean, you do appreciate the effect of light

for vision.

So when you wake up in a beautiful area, beautiful ocean,

light is essential.

The sunrise, the sunset, blue sky, beautiful mountains.

So that’s your conscious perception of light.

But light has a completely different aspect

that is independent of conscious vision

or image forming functions.

And that’s how it regulates many important functions

in your body.

I think the best that is well-studied and well-known

is your circadian clock.

And the word circadian comes from the word circa,

which is approximate and dn is day.

So it’s an approximate day.

Why is it an approximate day?

Because if I put you or any other human being

who have a normal circadian clock in a constant conditions

with no information about feeding time, about sleep time,

about what time it is outside,

you still have a daily rhythm,

but it’s not exactly 24 hours.

So it will shift out of the solar day

because it’s not exactly 24 hours

and hence the name circadian.

So just to ask a quick question about that,

when you say you have this about 24 hour rhythm,

how does that rhythm show up in the tissues of our body?

Great, so great question.

So it shows up at every level that we know, we studied.

It shows up at the level of the cell.

It shows up at the level of the tissue

and it shows up at your behavior.

The most obvious for you is your sleep-wake cycle.

You sleep and you’re awake and sleep at the 24 hour rhythms.

And if you measure the sleep-wake cycle of humans

who are maintained in constant conditions,

you will see that the period length

of the sleep rhythm on average is more than 24 hours

and a human’s is 24.2 hours.

So you’ll be drifting 0.2 hours every day

out of the solar day if you don’t get the sunlight.

So the sunlight adjusts that approximate day

to an exact day.

So now your behavior is adjusted

to the light dark environment or the solar day.

Okay, so if I understand correctly,

if I were to go into a cave

or I were to be in constant light

and I didn’t close my eyes in constant light,

that I would still sleep in one coherent bout

and I would still be awake

for more or less one coherent bout, maybe a nap.

But the total duration of my day, so to speak,

would be a little bit longer than 24 hours.

But if I’m in a condition like most people are

where the sun goes up and the sun goes down

and I have some understanding of that sunrise and sunset.

You don’t have to have the understanding.

You don’t have to have conscious understanding.

You have the detection.


So circadian photoentrainment is the word we use

in training the circadian clock to the photic environment

is completely subconscious.

You’re not aware of it.

It’s not like vision or image forming

where you actually know what you’re looking at.

So it’s all hypothalamic.

It’s part of the brain that is not consciously driven.

So you actually do not know when it happens

or when it doesn’t happen.

And that’s what we’ll get into

when I tell you why light affects your mood

and why sometimes people don’t know how to deal with light

to improve their mood, for example.

Okay, so this is a subconscious vision.



Before you tell us about how light impacts mood,

I’m curious, what is the relevance of adjusting this clock

from a little bit longer than 24 hours to 24 hours?

I mean, it seems like a small difference,

24 hours and 40 minutes or 24 hours.

Like what’s the relevance?

I mean, why should we care about that short difference?

So let’s do the math.

If you shift out 0.2 hours a day in five days,

you’re shifting out one hour.

So you’re literally one hour off

in your social behavior in five days.

In 10 days, you’re two hours off.

And if you’re an organism that is living in the wild,

shifting out of the right phase of the cycle,

you could either miss food or you could become food.

So it’s really essential for survival.

I think it’s one of the strongest aspect of survival

for animals to have the anticipation

and the adjustment to the solar cycle.

And for humans as well, when you say animals,

I’m assuming that applies to us.

Absolutely, yeah.

I see.

So even though it’s just a short bit longer than 24,

if that accumulates over days,

then you could find yourself very much out of phase

with the rest of your species, essentially.

So let’s say it’s 0.2 hours.

So in five days, it’s one hour.

In 25 days, it could be five or six hours.

You could be in New York and you’re feeling

as if you traveled from New York to London.

So you will be having jet lag in New York,

even though you didn’t do a jet lag travel.

So it’s very important for the adjustment.

And if we have time, maybe we could talk about

why this is important for seasonality,

because also it allows animals

to anticipate the change in season.

And the more you’re high in the North or the South,

the more that these weather changes occur very harshly

and you have to be ready for them.

And that happens in us as well.

All right, we will definitely get into seasonality.

Okay, so we’ve got this subconscious vision

that aligns us with the turn of the earth.

How does that work?

What is the machinery that allows that to happen?

And how does that machinery work?

Yeah, so we knew that in mammals, including us,

we are mammals, humans,

that the eyes are required for this function.

So if humans are born without eyes

or the optic nerves are damaged,

humans are not able to adjust to the solar cycle.

So we know that the eyes are required.

And since we thought we knew about the eyes a lot

before 2000, we thought-

What did you say, before the year 2000?

Before the year 2000, yes.

We thought it’s these photoreceptors in your retina

that allow you to see.

So in the human retinas,

there are two types of photoreceptors.

They are called rods and cones because of their shapes.

And these rods and cones simply take the photon energy,

which light is made of,

and they change it in a way to an electrical signal

that allow us to build the image of the environment

in our cortices.


Consciously, in this situation,

because it’s vision, right?

It’s image-forming vision.

It’s a visual cortex and associative cortices,

which allow you to build conscious perception

of the environment.

However, people have found,

including me with the work of David Burson

and Ignacio Provencio,

that there is a subset of ganglion cells.

The ganglion cells are the cells that leave the retina,

their axon, leave the retina,

and project to the brain.

So these were thought to only relay rod and cone information

from the light environment to the brain,

but we found that a small subset of these ganglion cells

are themselves photoreceptors

that were completely missed in the retina.

And these are the photoreceptors

that relay light environment subconsciously

to the areas in the brain that have and house

the circadian clock or the circadian pacemaker,

which adjusts all the clocks in our bodies

to the central brain clock

that allows them to entrain

to the 24-hour light-dark cycle.

So as I recall,

because I was a graduate student at the time,

in the year 2000,

there was this landmark discovery made by you,

Ignacio Provencio, David Burson, and others,

that these cells exist

that can communicate day and night information to the brain

in this very small subset of cells.

Since then, I’ve heard,

but maybe you can confirm or refute,

that this system that connects the eyes

to the rest of the brain

is actually the most ancient form of vision,

that this is probably the form of vision

that some early version of human beings had

before they had pattern vision,

before they could see colors and shapes

and motion and all that,

and that the same cells that perform this role

are actually similar to insect eyes.

I think I heard David Burson say once

that we actually have a little bit of the fly eye

in our eye.

What’s he talking about?

Yeah, so it’s really interesting, actually,

because these same IPRGCs we discovered,

they contribute a little bit to image formation,

and now work from Tiffany Schmidt

specifically have proven

that they do contribute to image-forming functions,

but they contribute to very limited aspect

of image formation,

so it fits your hypothesis

that these are an ancient photoreceptors.

The other thing that adds to that hypothesis

is that they are expressed in cells

that don’t have any modification

that make them look like photoreceptors,

so the photoreceptors that I told you about

that are important for vision, image formation,

they have very specialized structures

that allow them to pack these structures

with photopigments.

These are the photo-detecting proteins,

so they could detect a high sensitivity of photons

that pass through them.

These IPRGCs, these new photoreceptors

don’t have these specialized structures,

so they just really need a lot of light at the time.

We thought they need a lot of light to be activated,

so that’s why we think they are ancient,

and that’s why I think they adjust to ancient functions

that are as important as regulating your body,

circadian clock, to the solar environment,

to solar day, or to the light-dark cycle.

So you mentioned IPRGCs, intrinsically photosensitive,

so these are cells that connect the eye to the brain

that behave like photoreceptors, essentially,

and then you mentioned melanopsin,

which is the actual pigment that converts the light

into the electrical signal, more or less,

and my understanding is that melanopsin

was identified first in frog melanophores,

so does that mean that we have little pieces

of frog skin in our eyes?

So, honestly, David Bersin say you have a fly in your eye

because it sounds better.

The more accurate, I think,

is that you have a frog skin in your eye.

It’s not as catchy, but really, melanopsin,

really, the name melanopsin is from melanocyte opsin,

so it’s melanopsin because it was found

in the frog melanocytes.

You know the frogs can change their color

depending on light, and melanopsin drives this response.

So when Ignacio Provencio first discovered

these opsins in frogs, luckily he was smart enough

to see if they are expressed in the frog eye.

They were expressed in the frog eye,

and it’s what appears to be retinal ganglion cells,

which I told you, the one that connect the eye to the brain.

He had the insight to go and see

if they are expressed in the monkey eye,

and he found that they are also expressed

in what appears to be retinal ganglion cells,

and really, that what opened the field wide open.

Then David Bersin did the seminal experiment

where he went to the brain where the central oscillator,

the oscillator that drives circadian rhythm in the brain

called the suprachiasmatic nucleus

that has been known for many years

to receive retinal input,

and he labeled the cells that project there,

and then he found that even if you destroy rods and cones,

you could get light responses from these cells.

So you could imagine, he nearly fainted

when he saw that these cells can respond independent,

completely in the absence of rod and cone input.

Yeah, I’ll never forget reading those papers in 2000, 2001.

I was at the meeting in DC when Iggy, Ignacio Iggy,

we call him Iggy, showed this image of this,

basically what is frog melanophores in the human eye,

and everyone was like, oh my goodness, this is the thing.

And I want to get into how light

actually can control circadian rhythms in a moment,

but I think it’s worth mentioning now

that people who are pattern vision blind,

so people who cannot see, and no conscious vision,

but have eyes, many of them still have these cells,

these melanopsin intrinsically photosensitive cells,

and can essentially match, or entrain, as we say,

onto the light-dark cycle.

In fact, they possibly have no problems

in circadian photoentrainment.

They’ll have enormous sleep-wake cycle,

but they’re totally blind, but they are totally image blind.

And what’s really interesting is that,

and this story I heard from Chuck Seisler,

so I’ll give him credit,

that some of these people who are image blind,

usually they get dry eyes, and they give them a lot of pain,

and doctors used to think, oh, since they are image blind,

and they’re getting dry eye,

why don’t you just remove their eyes?

They’re not using them anymore.

And the minute they would remove their eyes,

they start having cyclical sleep problems,

indicating that now they are not entraining

to the light-dark cycle, and are having cyclical jet lags

when their clock shifts through the light-dark cycle.

That’s really interesting,

and I hear from a number of blind people,

in my various aspects of my job,

and a lot of them have issues with sleep, I think,

in part because they don’t realize

that they too need to see light

at particular times of day or night

in order to match their schedule.


Well, I think that’s a perfect segue for us

to talk about how light and viewing light

can impact our sleep-wake rhythms.

And then we will move into some of the other ways

in which light can impact other forms of bodily function.

Yeah, so I love the way you set it up,

because one of the most interesting and difficult aspect

of trying to educate people about light effect

on subconscious vision is that it’s subconscious.

So we’re all aware of what we think is intensity,

because we see the room.

But, you know, if you talk to people

who know how to take photographs and stuff like that,

they know that the intensity varies greatly.

But our system, because we have to see the same way

in very bright conditions and very dim conditions,

we’re not very good at estimating intensity consciously.

So when you try to tell people about intensity,

you really struggle,

because they think they know intensities,

but they really don’t.

You mean light intensity.

Light intensity.

So that the cones themselves have an incredible ability

to adapt to different light conditions.

So you can see at all different conditions.

Otherwise, it’d be a disaster.

You know, if you don’t change the setting on your camera

and you go from inside the room to the outside,

it becomes completely white.

You don’t see anything.

So if your cones don’t adapt to the environment,

then you’re not gonna be able to see in this room

and on the beach, right?

But the problem is your IPRGCs,

the cells that we talked about,

they measure intensity pretty well.

They really know what intensity is.

They have a very good linear measurement of intensity.

They don’t adapt as well.

They don’t adapt actually that much, to be honest.

So that tells you that subconsciously the system is used

to measuring light intensity in a natural environment.

Because when you’re in a natural environment,

you don’t have industrialized lighting,

then your system is functioning very well.

But now when we change these environments,

we could really mess up ourselves.

So you have to teach people how to understand intensity.

And that’s something that you have to explain to people.

And I think I love to do it myself.

I do it in what is called the lowest amount of light

required to allow you to see comfortably.

So you have to do this as a fun experiment.

Okay, so explain to me how this goes.

And maybe we could break it up in the day

into three or four parts.

So let’s say, assuming that most people wake up

in the morning, as opposed to night shift workers, et cetera,

we could talk about later.

But you wake up in the morning.

So let’s divide the day into quarters.

What is the proper way to interact with light

in the first part of the day?

So I honestly think the easiest thing is waking up.

Get as much light as you can.

Into your eyes.

Yeah, it’s really nice.

Your system is primed.

If you’re entrained, it’s primed to get light.

The sun should be out.

Most animals in the wild,

they actually seem to track the sun.

The sun has a huge influence on life on Earth.

It’s actually, life on Earth is because of sun.

So that’s easy.

In the morning, when you wake up, you need light.

Okay, so what is the behavioral practice

that you recommend?

Let’s say somebody is in a condition

where there’s a lot of cloud cover.

Is it important to get outside?

So I have to tell you, the cloudiest day

is gonna be much more brighter than your room.

You could ask any photographer.

A cloudy day, unless it’s really dark, dark clouds,

usually cloudy days have much more bright outside

than inside the room,

even when you have good lighting inside the room.

So I think in the outside is usually,

even when it’s cloudy, you’re gonna get enough intensity

to help you adjust your cycle to the day-night cycle.

So how long do you, these are general rules of thumb,

but how long do you recommend people go outside?

So if you do it daily, you possibly need very,

if you do it daily, because remember,

this thing is gonna happen on a daily matter.

So it’s like 15 minutes.

So the clock is tracking it on a regular basis.

Absolutely, it’s photon counting, it’s tracking.

I would say 15 minutes.

If you don’t do it daily, you may wanna increase it.

And we’ll talk about when you travel, what you could do.

But yeah, 15 minutes should be fun.

You do it more, it doesn’t hurt.

And through a window, my understanding is that

through a window, it dramatically decreases

the amount of light energy coming in.

It depends of how thick the windows are

and how dark they are.

But it’s also nice to go outside and to feel the season.

Sunglasses off.

I don’t use sunglasses.

Yeah, but you have the Jordanian full pigment, you know?

So yeah, whereas my eyes are very sensitive, right?

But I personally, if I’m in the shade

or if it’s not incredibly bright,

I try to, especially in the morning,

but I’m also an early person,

so we have to differentiate between early and-

What time do you wake up?

I wake up at 4.30 in the morning.

But the sun isn’t out yet.

It’s not out yet, so it’s-

So what do you do?

You turn on artificial lights?

I usually don’t turn on artificial light

because I know the sun is gonna come up eventually,

but that’s why I don’t like the change

in the timing that they do.

Wait, but what do you do between 4.30 a.m. and 7 a.m.?

I look at my computer and my phone,

so possibly I get enough light.

But in reality, as long as you let your body

get the morning sunlight, which I think is really,

to me, and there’s no evidence,

but to me, if you look at all animals, plants,

this morning sunlight seems to be very important.

And we don’t have experiments to show it,

but I have a gut feeling that it has

a huge impact on humans.

Well, Jamie Zeitzer’s lab at the Stanford Sleep Lab

has shown that these early morning light flashes

can adjust the total amount of sleep

that one will get.

It makes it easier to get into sleep.

Absolutely, absolutely.

Okay, so-

And Ken Wright also did this beautiful

camping experiments that showed-

Maybe you should describe those

because those are beautiful experiments.

They are beautiful experiments.

He took these college students

that had a late onset of sleep and late waking time,

and then he said, let’s go camping

and just don’t use any artificial light,

and you could go to sleep as late or as early as you want

and wake up as late as early.

And he found a huge shift in their sleep pattern

just by exposing them to the light-dark cycle.

I mean, so-

And it lasted.

And it lasted.

Even after they came back.


I think it was two days of camping,

reset the circadian clock.

Seven days, but it lasted, yeah.

It’s pretty amazing, yeah.

It’s really incredible.

Okay, so get bright light of some sort

early in the day, ideally sunlight.

Even on a cloudy day,

it’s going to be brighter than indoor light.


So that’s easy.

Okay, so then-

And the other thing that I would like to mention to people,

if you think it’s very dim outside,

let’s say it’s very cloudy, stay longer.

So remember, intensity is only one component.

Duration is also important

because remember that the circadian system,

it’s not like the image system.

In the image system, you have to change every second

because you’re looking at different objects.

You have to change your perception.

But for the circadian system,

it’s trying to figure out,

where am I in the day-night cycle?

So the more you give them the information,

the better you are.

So if it’s very bright, you don’t need a lot

because it’s clearly going to make you fire like crazy.

But if it’s not bright, stay longer.

Stay for one hour.

You know, have your coffee outside or something like that.

It’s just going to help.

I think you said something extremely important,

which is that this circadian system

is trying to figure out when you are in time.

Not where you are in space.

Sorry, I said where you are in time.

I meant when you are.

Oh, no, no, I wasn’t correcting you.

I just meant that I think fundamentally

that’s the incredible thing about this system,

that you have this clock, this 24-hour clock in your brain,

but it needs to be synchronized to the outside.

So could we go a little deeper

into this circadian setting behavior

and come up with some general rules of thumb?

So let’s say it’s a very bright day, extremely bright.

No clouds, sun’s out.

You said 10 minutes, 15 minutes.

And I’ll tell you, if you’re sensitive,

you don’t even have to go in the sun.

You could be in the shade.

There’s going to be so many photons out there in the shade.

It’s going to be perfect.

You don’t even have to see the sun.

You don’t have to have the sun.

You know, it’s great for vitamin D.

That’s a different story.

You could do this for your skin and protect your skin.

That’s not my area of expertise.

But for that effect on the circadian system,

as long as you’re outside in the shade and it’s a sunny day,

10 to 15 minutes should be ample amount.

Okay, and then let’s say it’s kind of overcast.

You know, it’s not particularly bright

or there’s solid cloud cover,

but obviously the sun is out, but it’s not as bright.

How long do you think it would take to set the clock?

10 to 15 should be sufficient.

Stay for half an hour, stay for 45 minutes.

If it’s very dark cloud, yeah, stay for longer.

Okay, and if for some reason one finds themselves

very far north and it’s very, very dense cloud cover,

how long and at what point should somebody consider

using an artificial light source to mimic the sunlight?

Yeah, honestly, this is where we don’t have

a lot of information still,

because this is where we’re going to discuss this

maybe in more detail,

that if you put humans in artificial conditions,

the circadian system is very sensitive to light.

But in reality, in the real environment,

light also is affecting other aspects

that are independent of the setting

of the circadian pacemaker.


And these, which we call the direct effect of light

on mood, for example.

So that is very hard to figure out

what intensity you need to use.

And we haven’t done enough experiments

because the system has been discovered just recently.

But I would say if you use bright light in the morning,

and I mean, it’s hard for me to give numbers,

it can get complicated.

But yeah, I mean, if you’re, honestly,

if you’re that far north and you’re in the winter

and you want to get, make sure you don’t,

use these light boxes.

I would suggest that personally, but that’s it.

I use, it’s actually not designed for circadian setting,

but I have a 930 lux light pad

that I bought and I bought it.

They’re very affordable compared

to the dawn simulating lights,

which are quite expensive, frankly.

And I put it there.

And so I just basically, when I wake up in the morning,

I use that until the sun comes out.

And then I make sure once the sun is out, I go outside,

but I keep that thing on all day.

And I don’t know if that’s good or bad.

Is it good or bad?

I honestly, I don’t think being exposed to bright light

in the day is going to ever be bad

because really if you’re outside in the day,

unless, you know, the worst is going to happen

if the temperature is very high,

your body’s going to say, don’t dehydrate and go to sleep.

So you could tell actually sometimes when it’s very hot,

the more you get exposed to bright light,

the sleepier you feel in the afternoon,

which is counterintuitive.


And that’s to protect us, you think, against dehydration?

I think if you think about the human evolution

from near the equator in the,

between noon and a certain time in the afternoon,

it would have been very hard for you to maintain

physiological homeostatic function,

being active at this very high temperature time.

So I think napping was a way,

that’s why I think it has a major function,

which is still,

napping was a way to somehow take you away

from that dangerous zone.

And maybe that’s why people in the North,

they say in the winter, we can’t wake up in the morning

because they don’t have this long light.

So they sleep it more at night.

But in the summer, they say,

we feel like we can’t go to sleep.

We have to put all these dark curtains.

So I think, you know,

venturing that much up North has been,

came up with problem because evolution was used

to a certain light environment

that was completely changed with a human.

With other animals, I think, that lived there longer,

they have come up with very interesting adaptation

of actually measuring even very small changes

in the light intensities that still occur.

So even if you’re near the poles,

even though it’s always light,

but there is a change in the light intensity

across the day and night cycle.

So your system, if it’s linear,

and remember, I told you that IPRGCs are incredibly linear,

can still measure, oh, this is lower light than higher light,

if the organism has the ability to do that.

I see.

You know, it’s interesting.

I’ve spent so much time learning from you, fortunately,

about these cells,

and yet I never really appreciated until now

how, on the one hand, they are tracking the amount of light

to understand when we are in time

relative to the 24-hour cycle,

but also that you keep mentioning

this linear measurement of intensity,

that they really are trying to figure out

when we are in time by measuring the intensity of light.

And of course, the sun is the most intense source

of light available to us.

So, okay, so I think we’ve nailed down

that first part of the day.

Basically, it’s get 10 to 30 minutes,

depending on how bright it is,

and try and do that as often as possible

to give the system a regular-

Daily is the best.

This system is really about,

and you’ll see that even for the effect on depression,

it’s about multiple days.

So you don’t have to worry if you missed it one day,

you know, stay longer if you want,

but if you’re in a hurry and you want to do other stuff,

that’s a great recommendation.

So you might want to compensate with some extra time

if you missed a day or two.

And this is why I’ve heard you say before,

it’s entirely possible to get severely jet-lagged

without traveling.


Simply by staying in, being on your phone too much,

not getting the sunlight.

And you saw this during the pandemic.

A lot of people mentioned

that their sleep-wake cycle suffered a lot.

Because if you’re not going out,

and if you’re staying at home,

and you don’t have big windows,

and you’re waking late, waking up late,

and then you’re using very bright light till late at night,

your body’s going to shift.

And now your day is going to start instead of like,

really when the sun comes up,

let’s say at six o’clock in the morning,

it’s going to,

your day is going to start at 11 o’clock in the morning.

That’s what your body’s going to think

is the beginning of the day.

So then you’re not going to be able to sleep

at 10 o’clock at night,

because now that’s really,

for your body, it’s completely different timing.

And you could see this happen during the pandemic

at a very high scale.

People get delayed in their sleep-wake cycle a lot.

There is this idea of chronotypes

that we all each intrinsically have a best rhythm

of either being a morning person,

you call yourself an early person,

or a night owl,

or more of a kind of standard,

to bed around 10.30, up around seven type thing.

And I think there are now good data,

correct me if I’m wrong,

from the National Institutes of Mental Health and elsewhere,

showing that the more we deviate from that intrinsic rhythm,

the more mental health issues

and physical health issues start to crop up.

So there is great data on this,

and there is a couple of things that complicate this.

The first is the people who usually are late.

They tell you that the society doesn’t accommodate them.

What, by late, what do you mean?

People that wake up late and go to sleep late?

Go to sleep late and wake up late.

They have an overwhelmingly higher level

of depression and failures.

I mean, clearly, I mean,

the reason that people say sleep early,

wake up early, you’re better,

because human notice that people who wake up,

go to sleep early and wake up early,

they do better in life.

They notice that.

They just perform better.

They perform.

But the question is, is that intrinsic to the system,

or is that society?

Because society start things usually early or late.

That’s a hard question.

We discriminate against late risers.

In a way, we discriminate, right?

But the other explanation is Ken Wright’s experiment.

These late risers,

if they were truly chronotypically late,

why would they shift so easily

when you put them in the,

if you were really chronotypically late,

and there is a phase relation

between the light-dark environment

and your circadian clock,

then doing this camping experiment

should not have caused much changes,

because it’s not that, you know,

light is gonna affect you in a certain way.

It’s that this is the relationship

that your body decided,

that I’m a late sleeper, late waking.

So I’m honestly, I’m still unable to figure out

how much of this late waking up

is controlled by the light environment,

and how much is intrinsic.

I’m sure there are differences,

but are they as big as we see in the environment?

Because you have people that go up to sleep

at 7 p.m. and wake up at 1 a.m.

These are clearly advanced phase.

So people that go to sleep at 7 p.m.

and wake up at 1 a.m.

and feel good doing that.

I’m not so sure they feel good,

but a lot of the time you talk to people,

they say they are high achievers,

but they suffer because, you know,

they wake up, they go to 7 p.m.,

wake up, advanced phase sleep syndrome,

they call it, they call it a syndrome.

But, you know, but then you have people

who would not be able to sleep till 5 a.m.

and not be able to wake up till 3 p.m., right?

And I’m not so sure that the circadian system

is that variable in the human population.

I mean, clearly there are maybe some genetic factors

that make a small percentage of, like,

everything with a bell shape,

but I think most of the time

the light environment may play a role.

And once, as we’ve talked about,

this is a long-term effect of light.

Once you get into a rhythm,

and I don’t mean it as a pun,

in reality, once you get into a rhythm,

it’s hard to break out of that rhythm

because if you start sleeping late

and waking up late,

you’re not getting the morning sunlight.


And so you’re just gonna be late.

And if you’re like me, waking up early,

you’re getting the morning sunlight.

You’re getting what Zeissler said,

I said his last name wrong,

the one in Stanford who did the-

Oh, Jamie Zeissler.

Zeissler, yeah.

He actually worked for Zeissler.

So Zeissler and Zeissler.

Yeah, there are a lot of Z’s and I’s in their names.

Both phenomenal scientists.


What it seems to me is the case

is that the only way to really know

if you’re meant to be an early bird,

as they call it, an early person or a late person,

or somewhere in between,

is to get morning sunlight

and figure out whether or not that makes you feel better.

And to understand,

to be educated about how to measure intensity,

how to measure, I put it between quotation,

because you either get a measuring device,

but you cannot depend on your eye to measure intensity.

Okay, so how do we do that?

Because you keep coming back to this,

so that tells me that it’s important.

So there are apps, like free apps,

like Light Meter, where you can walk around

and hold the button down

and see how many lux are in the environment.

These are complicated

because you have to point them to specific regions.

So how do people start to develop an intuitive sense

of the measurement of intensity?

Yeah, I think at one point I posted on Instagram

how I keep my nighttime at home.

And I found out that my night vision is very strong.

So I found out that I, especially in the winter,

I only need candle lights.

So I literally use these tea lights

and I put like 15 or 20 of them.

How romantic.

And it’s so nice.

I could see it clearly doesn’t affect my circadian system.

You and your cat.

And my wife.

And your wife, of course.

And it’s just great.

It’s just great, right?

But I don’t expect people

to have the same night vision as me.

So the simple, I mean, I tell people, do the experiment.

So if you put three or four lights in your room,

switch two, sit for 15 minutes.

Switch two off.

Switch two off.

Let’s say you’re using five.

And see, after 15 minutes,

you will not recognize you switched these two off.

My gut feeling is that most people would need

at least 10 times less light than they use at night to see.

The problem people use it

because most of the time they didn’t see

the morning sunlight.

They are actually hungry for light without their knowledge.

So they come switch all these lights on,

but at the wrong time.

Because they woke up late.

Okay, now I understand.

So this morning light viewing

goes way beyond setting your clock.

It’s also a way to determine

how little light you need later in the day.


And we’re going to talk about this in a moment,

but how little light you get later in the day

is a very strong determinant of things

like when you will wake up,

whether or not you wake up feeling refreshed, et cetera.


And that’s why-


I’m going to break it on your show, Andrew,

that I’m going to tell you,

I think there is something else

that people need to think about,

which is the tripartite model.

That this model incorporate three components,

which we’ll talk about in details,

that allows us humans and all animals

to incorporate the circadian clock

and its relation to light,

the homeostatic drive,

and the direct effect of the environment,

which includes stress, light, all kinds of stuff.

They have to be incorporated together.

If you think, that’s what I think right now,

if you think of one alone,

you will always miss something.


When you think of them as a whole,

things really become clear.

It’s actually quite amazing.

Okay, well, we will definitely want to hear

about your tripartite theory

and go into detail about this homeostatic mechanisms.

I want to make sure that for people

who are thinking now, I’m sure,

about light and how it impacts them.

So the morning light viewing behavior,

I like to think we’ve tacked down clearly.

And thank you for that,

because there’s so much information out there.

And I’ve tried to relay that information.

Of course, you’re my primary source

for all things circadian,

as well as Jamie and others, of course,

and Matt Walker.

But I think you’ve made that very, very clear.

Now, let’s say I’ve gotten my morning sunlight, okay?

Made my bright artificial light.

And throughout the day, you said to get a lot of light.

So I’m working at my desk.

Maybe I’ll go out during the day a few times,

but I’m working at my computer.

I’m doing things.

Is there anything about light viewing

in the middle of the day that people should keep in mind?

Or can they just sort of freestyle it,

depending on what they’re doing?

Most people are not in a dark room throughout the day.

My gut feeling, if you got your morning sunlight,

you walk from your car slowly,

or you walk to work,

you didn’t wear sunglasses

when the lights were still dim in the morning,

that you could freestyle it.

That even if you don’t get a lot of light,

there is a way to just, in the day,

you don’t have to just worry

about getting a lot of bright light.

But personally, I like to do that.

So I go out at lunch and have my lunch outside as well.

This reminds the body that here it is even brighter now.

But the evidence is that you could literally

help your circadian clock

by giving lights at dawn and dusk.

But again, if you think of the tripartite model,

this may be important versus circadian clock,

but is it important for your mood?

So that’s where I think you need,

or the homeostatic drive.

So that’s where you need to think about it.

So for the clock, for entraining your clock,

you literally can entrain it only by the dawn sunlight.

You actually don’t need dawn and dusk.

People even forget that.

Yeah, and I appreciate that you’re distinguishing

between circadian effects and other effects of light.

You’re being very precise, which is appreciated.

Until we hear about this tripartite model,

which we will cover, for the sake of the discussion,

let’s treat the light viewing behavior

as what are the benefits or drawbacks of viewing light

for all biological purposes, not just circadian setting.

So in the morning, it’s clearly going to set the clock.

And then during the day, if I understand correctly,

the idea is to get as much bright light as you can,

because you’re feeding, it sounds like,

a sort of light hunger.


I see.

I love this way to put it.

I think there is a weird light hunger,

considering that we’re not photosynthetic organisms.

There is a weird light hunger in animals

that they need to measure, they need measure.

And I think that relates to the season,

because the whole reproduction cycle of animals

is going to depend on the availability

of food in the environment.

And if you don’t know when the season’s going to happen,

they don’t have calendars,

it’s going to be very hard to survive.

So I think that’s why we have this light hunger.

That’s a major hypothesis, it’s not being tested.


So then afternoon and evening start to approach.

So I’ve had this weird experience,

maybe you can psychologically

or biologically diagnose me now, Samer.

So where if I go into a movie in the afternoon,

like a matinee, and I come out and it’s dark,

I notice a significant drop in my mood

and my ability to go to sleep.

Whereas if I get some view of the light in the evening,

it doesn’t have to be the sunset,

although sunsets are nice,

but I get some light pulse in the afternoon

that I have no trouble whatsoever falling asleep.

And it happens in a daily, on a single time to watch them?

More or less.

That’s interesting.

And then you mentioned the camping experiment,

where when they went camping,

they’re seeing the sunrise and the sunset.

So what should people do in the afternoon slash evening time

in terms of their light viewing behavior?

I mean, the best thing to do

is to let the natural light creep in into darkness, right?

That would be the best.

But clearly that would be inefficient.

You wanna go home, you wanna read,

you wanna talk to your kids,

you wanna talk to your family.

So I think, you know, it’s nice to extend the day.

I don’t think that’s wrong.

If you somehow can block that light

from affecting your circadian clock.

So should people use blue blockers in the evening?

I personally do not like any blockers

that take a single wavelength of light.

Because again, if you think of a holistic approach,

yes, the blue blocker is gonna prevent you

from affecting your circadian clock very much.

But then your vision is gonna be distorted

because we always see in full spectrum.

The sun has this beautiful spectrum, right?

And then when you start seeing without the blue,

things look yellow and it can get really weird, right?

I mean, so I personally, I’ve tried the blue blocker

and I couldn’t even wear them.

I thought they were just really horrendous, to be honest.

Well, along the lines of blue blockers,

I think a lot of people mistakenly wear them all day long.

Oh my God, that would be very bad.

A lot of people do that.

A lot of people do that.

They think that blue light is bad.

I think that the concept of blue light being bad

led to a lot of product development.

And a lot of people are just assuming

that viewing blue light is what was giving them headaches

when in fact it might’ve just been looking at screens

at close distance.

So here’s the problem, right?

I mean, the blue light got the bad reputation

because people who gave a pure blue light

showed that it caused a huge retinal damage.

But again, if you’re using blue light in its pure form,

it has a lot of energy because it’s shorter wavelength.

But we’re talking about full spectrum light.

There are ways now where you could change the spectrum

of the light and keep it white between day and night

and change the content of the color without you noticing.

So you don’t even have to affect your vision.

So how would you go about doing that?

So you just lower the level of the blue light.

You don’t have to eliminate it.

So just dim the lights.

Dim the blue, but then increase the yellow,

but keep all the colors in a certain white.

So you could have different warmness of white.

And people know how to do this.

Physicists know how to do this.

People who work with light know how to do this.

Well, maybe somebody in the wellness slash,

I don’t like the word,

but biohacking or optical community will do this.

I think it’s really important.

I see so many people wear blue lockers.

I don’t know why they love it.

Well, I think they’re just uninformed.

I think, frankly-

And to be honest, it’s easy, right?

It’s easier to explain to somebody.

If IPRGC respond mostly to blue, remove blue,

you’ll be fine, right?

But that’s not as simple as that

because they also receive Rhode Island cone input.

And we could go into details

that’s boring for your listeners,

but it also affect the adaptation properties

of the whole retina.

So you don’t want to do something so drastic

that you take just one color of the spectrum.

It just seems very counterintuitive to me, to be honest.

You’ve told me before as well

that just because these intrinsically photosensitive

circadian setting ganglion cells respond best to blue light,

if the light is bright enough,

because they also get input

from other components of the eye,

it doesn’t matter if you block the blues.

If you’re looking at bright light at night,

you’re going to disrupt your circadian cycle.

And that’s why I didn’t want to go into the boring details,

but themselves, the photoreceptors

have a wide range of responsiveness.

So they are most sensitive to blue light,

but that doesn’t mean they don’t respond to green light

or to shorter than blue light.

They respond to very, very wide spectrum

with different sensitivities.

So unless you understand the system,

just removing 480, I don’t think it’s going to do anything.

480 nanometers, yeah.

So your home is a cave at night, basically.

It’s a nice cave.

It’s a nice cave with candles, right?

And you and your cats and your lovely wife,

who I know who’s also a phenomenal scientist

in her own right.

Yeah, she is.

She is, but you do keep your home

quite dim to dark at night.

In fact, I did go to meetings

with some of my friends who work on this,

and they really struggled with me.

They said, we could have broken our legs

living in the same light environment that you do.

So I am an extreme, but I measured it for myself,

and I asked Reji, my wife, if she’s okay with it.

She also liked the dimness.

Both of us can see well in dim conditions,

and that helps us a lot.

But I think you have to measure it for yourself.

You really have to do.

It’s a very simple experiment.

Just try to dim the light as much as you can.

I call it the minimum amount of light

you require to see comfortably.

And that’s how you want your environment ideally at night.

This is what I think is the game changer.

If you reach to a level where it’s just barely,

you’re literally on the cusp of seeing uncomfortably

versus seeing very comfortably,

you are gonna be very much better than,

I don’t like to make it completely dark.

I think complete darkness induce anxiety in humans,

to be honest, so I don’t like complete darkness.

In fact-

Kids don’t like complete darkness.

They like a night light.

Even nocturnal animals don’t like complete darkness.

I mean, we have studies in animals that are nocturnal

that if you put them in complete darkness for several weeks,

they have severe anxiety and depression-like effect.

So keep the light dim.

Use red light that is very dim.

If you wanna keep the room for sleeping,

red light that is very dim has very small effect

on circadian clock.

And below 10 lux of red light

literally doesn’t affect sleep at all.

So there are ways to do it.

It’s just we need to educate the public.

And I feel like you literally need a whole lecture

to just explain to the people how to deal with light

because it’s not as simple as people think.

Well, that’s what we’re doing here.

We’re stepping through it piece by piece

and the reason we’re doing that

is because it’s not as simple as saying

just block blue light or get a lot of light during the day

and minimal at night.

I mean, just to put it in perspective to tell it,

we only have three different cones in our retina

that respond to three different colors.

We call them red cones for simplicity,

green cones and blue cones.

Yet we have only three of these,

but we could see massive palette of colors.

So that tells you something.

If the system was just simply about a single color

and it’s just removing 480 or just blue is sufficient,

then we should only see in red, yellow and blue.

We shouldn’t see all these different hues of color,

but because the system is not that,

we see all these different colors.

And that’s why it’s important to remind people

that the white light is made of many different colors.

It’s actually like the rainbow.

That’s why you see the rainbow.

It’s made of many colors.

White light is never truly white.

It’s made of a lot of different colors.

It’s like the Pink Floyd album cover.

Exactly, exactly, exactly.

So dim at night, maybe dim red light ideally

or candle light, find that minimum required light level.

Just make sure when you lower the light,

sit for at least 10 to 15 minutes,

let your system adapt.

Because if you had a bright light and you switch it off,

surely you’re going to suffer

because your system didn’t adapt it.

It was used to very bright light.

So you want to engage your rods,

which take a long time to dark adapt.

So that’s why I tell you, just wait a little bit.

Don’t just switch it off, I don’t see, put it on.

Put it off, sit down, wait for 10 minutes,

ideally 15 minutes, and then see how you see.

And then once you do that,

you will notice that actually, yeah,

I could see quite well, even with much less light.

What do you do regarding screens?

Yeah, that’s the hardest thing.

Again, I mean, there are beautiful programs

that change the whole intensity and color of the screen.

These could help dim your screen at night

to the lowest part.

I mean, yes, you won’t see it when you wake up

in the morning, but then you can increase the intensity.

So try to decrease.

I mean, just what we were talking about.

Think of light intensity, duration, color,

and time of day.

You really have to keep these four things together, right?

We’ve roomed together at a couple of meetings

from time to time, no longer,

because one of us, not to be named,

has a severe snoring issue

that made the other one pseudo-homicidal.

You can guess who that was.

But I’ve seen you check your phone after dark

once or twice, and you did it by sort of

pointing your phone away from you, right?

And actually, I’m sort of half joking,

but I, and you dim it quite a bit.

I’m sort of half joking, but it actually makes sense

that if you shine a flashlight in your eye,

it’s much brighter than if you shine a flashlight on the-

Light only go in direct light.

So if you just look on the side,

most of the light is gonna go this way,

and you’re only seeing this way.

So you’re, okay.

So, and as silly as that might seem to people listening,

I mean, what it means is that

getting bright light in your eyes at night

is something that you really want to avoid,

but there is the reality that-

And even when I check sometimes,

if I have something and I check it so fast

and switch it off so fast,

so I’m also aware of the durations.

Not my messages.

I’m also aware of the duration, right?

So duration, intensity, color, and time of day.

Ideally, I should not check iPhones and iPads.

I don’t use iPad at night

because it’s hard to lower it enough

because it’s a huge, but even my iPhone,

I try not to use it at night.

And like once it becomes 8.30 or nine,

I don’t look at it at all.

Unless it’s World Cup or Euro Cup,

in which case Sam is on 24 hours, everybody.

That’s only every four years.

It’s a big soccer fan.

All right, this has been incredibly, no pun, illuminating.

Let’s talk about the relationship between light

and some of these other non-circadian

or pseudo-circadian effects.

And we will try and link those.

But you had a, what I consider absolutely landmark,

beautiful paper published in Nature a few years ago,

showing that if you disrupt the exposure to light

or the timing of the exposure to light,

that there are dramatic effects on the stress system

and on the learning and memory system.

We could talk about each of those separately or together.

What are the effects on stress

and the effects on learning when light-viewing behavior

and sleep-wake cycles are disrupted?

Yeah, so just to remind you, you know that,

but to remind your listeners that I was trained

as a circadian biologist.

So I really was indoctrinated into thinking

that light has to affect the clock,

which then cause all these different effects.

So that’s what I believe.

That’s my dogma.

That’s what have made me really happy.

And then Tara Legates and Kara Ultimus joined the lab

and said, and we started discussing a lot of data.

And we said, what if there is a direct effect of light

that we’re missing, independent of the circadian clock?

So this is not an easy question to answer

because as we’ve been talking all along,

light affects the circadian clock.

So how could you give light at different times of the day

and not mess up the circadian clock?

Luckily, we came up with such a way.

And that’s why it was important to do these experiments

the way we did them.

And we proved that this light-dark cycle

does not disrupt the clock.

There is still a circadian rhythm

and does not cause sleep deprivation.

And yet, surprisingly, if you give light

at the wrong time of the day,

even without disrupting the circadian clock

or without causing sleep deprivation,

as you mentioned, you get huge mood changes

in the organisms and you get learning deficit.

So this really, and at the time,

people have really hit us hard.

I mean, it was really hard to publish this work.

And you could, yeah.

Well, it came out in nature.

So in the end, you prevailed.

But I want to make sure that I understand.

So you’re saying that, yes,

there are effects of light on the circadian rhythm.


Meaning sleep and wakefulness.


And their timing.

However, there are direct effects of light on mood

that can be dissociated from the effects

on sleep and waking.

So if I interpret that correctly,

that could mean that when we view light

and how much light could make us feel happier

or less happy or even depressed,

stressed, learning, et cetera.


Even if we’re sleeping and waking up

at the appropriate time.


I mean, eventually, because we’re talking

about the whole system,

eventually when you start having the other problems,

you also develop sleep problems.

But you’re absolutely right.

And in fact, now, research from Diego Fernandez

in the lab have found that now we know

that they actually require different brain regions.

So we don’t only have a theory,

we don’t only have a light environment

that showed they can be dissociated.

We know that they use completely different brain regions.

So the SCN that I told you about earlier,

the place where the central pacemaker is,

the one that receives direct input

from the retina through the IPRGCs

to adjust your circadian clock,

is not the area that receives the light input

for mood regulation.

It’s a completely different brain region.

What’s the brain region called?

So the brain region,

we called it the perihavineolar nucleus.

I’m not so sure how good or bad the name,

but it doesn’t matter, it’s the PHB.

And what’s really amazing,

this region also receives direct input

from the IPRGCs, but projects to areas in the brain

that are known to regulate mood,

including the ventral medial prefrontal cortex,

which has been studied for many years

to be impacted in a human depression.

So just by this amazing serendipity

to find that a region that is so deep

in the advanced brain,

like the prefrontal cortex is your executive brain,

one of the most elaborated in humans,

to see that they receive input

from this ancient photoreceptor was stunning to us

and told us how much we didn’t understand

the importance of light on human behavior.

So how does that finding inform daily protocols

for you or for other people?

I realize you can’t leap to always from one paper

to daily protocols,

but if light indeed does control prefrontal cortex,

executive function, learning, stress, and mood,

and let’s say I’m waking up each morning

and I’m sleeping, what should I do differently?

That’s why we came up with the tripartite model

because yes, we could think about just adjusting the clock

with lights in and being dark throughout the day,

but that may not be important

for your whole physiological function.

So now if we include these other effects of light,

that’s why I prefer to still get a lot of light in the day.

I don’t want to be in very dim light condition

throughout the day.

I see.

So even though it doesn’t affect your clock,

as you beautifully said, Andrew,

it may affect your mood and learning and memory.

It may affect your alertness level,

which is going to allow you to learn better.

It may affect your homeostatic drive.

Maybe your homeostatic factor will go higher

so you could sleep earlier.

So it’s important to think of light

as stimulating all these brain regions,

which means it’s producing more activity,

which in reality,

this is how people think of the homeostatic drive,

that the more active you are,

the more the homeostatic drive is built up,

the better you sleep.

So that’s why we came up with the tripartite model,

because as a circadian biologist,

I only thought of light through the circadian clock

affecting behavior.

As a sleep biologist,

they only thought of the homeostatic drive

affecting sleep, affecting behavior.

And for people who study light for vision and other things,

they thought only of the environmental input.

But now if you put them all together,

you get with this tripartite model

where it’s really mind-boggling and it makes so much sense.

The organism doesn’t want to depend on a single component,

but if you could incorporate these three together,

you could have a beautiful system that is well adapted.

So let me tell you the sleep-wake cycle, right?

So we know there is a homeostatic drive to affect sleep.

We’ve had beautiful talks about that.

Which is basically the longer you’re awake,

the more you want to be asleep.

So that’s your homeostatic drive.

We’ve talked about the circadian influence of sleep

and the fact that light-dark cycle

affect the circadian system,

which eventually affects sleep.

So these two components are well understood.

Now the third factor is your direct light

or environmental input.

How much stress, how much light you get from there

also can highly impact sleep.

So even if you have a good circadian and homeostatic drive,

if you’re getting light at the wrong time of the day

or if you’re being stressed and thinking,

then your sleep is going to suffer.

So you have to think of the three together

to have a beautiful sleep-wake cycle.

And that’s why we came up with the tripartite model.

The same thing happens with feeding.

I could beautifully put it to people.

Your hunger, your energy level

is measured by the arcuate nucleus.

Your daily intake of food is, again,

dependent on the SCN and light-dark input.

We found that if food is not available,

there is yet a third input that is not dependent on the SCN,

not dependent on the arcuate,

depending on a completely different brain regions.

So the animal can actually start looking

or the human can start looking for food when it’s scarce,

even at time when they are not supposed to be active.

So that’s how the organism think

they have to evaluate multiple inputs

for them to decide what is the best physiological outcome

at that moment, at that season.

I see.

So I want to get into arcuate and feeding,

but just to make sure we can keep our hands

around this tripartite model.

So if I understand correctly,

we’ve got the circadian influence,

then you’ve also got the drive to sleep.

Actually, one of the ways that I think

that can be best understood is

if somebody ever pulls an all-nighter,

they get tired around 11 or 12 or so,

and then very tired around 3, 4 a.m.,

but then even if you stay up,

sometime right around 7 or 8 a.m.,

your normal wake-up time, you start to feel alert again.

And that’s because the sleep drive is extremely strong,

but there’s a circadian rhythm

that drives wakefulness in the morning.

Okay, so those two are the components.

Before we get into the feeding component,

I want to talk about these direct effects of light on mood,

okay, Diego Fernandez’s data,

and this perihabenular thing.

So let’s just, for the moment,

set aside the tri part of the tripartite model

and just focus on what are the direct effects

of light on mood?

And the way that I interpret what you’ve said so far

is that the protocol that emerges from this,

if one is trying to optimize their mood,

is yes, see light, view light, I should say,

early in the day in order to set your circadian clock,

maybe also in the evening as well.

And of course, avoid light at night,

get it as dim as possible.

However, you said it’s also a good idea

to get as much bright light during the day

as you safely can in order to improve your mood

independently of regulating your sleep-wake cycle.

And that’s a hypothesis.

Here’s the problem where it’s not going to be

as satisfying as the circadian,

is that as you know,

this brain region has been discovered very recently.


The perihabenular region.

We’ve known about it a long time,

but nobody knew what it did.

So we knew about the habenula,

but that’s why the name is confusing.

It’s actually not the habenula itself,

it’s the perihabenular.

Oh, near the habenula.

It’s near the habenula.

Why don’t you just call it the Samar-Hatar nucleus?

I should have, I don’t know why I’ve done that.

Maybe because if you do that, it’s not okay.

Okay, so for here ever after, the perihabenular nucleus,

we should probably call it the Hatar-


How about Hatar-Fernandez-Burson nucleus?

Okay, this is like nerdy science attribution stuff,

but I’m just going to call it the Hatar nucleus.

Wikipedia, line it up.

Okay, so this structure is taking light

and independent of sleep rhythms and circadian rhythms,

it’s driving changes in mood.

How does it do that?

Is this through the dopamine system, the serotonin system?

We still recently,

we haven’t identified this region very well.

We don’t know what light does to it.

We don’t know how it interacts.

So this is an area that is ripe for discoveries

and we’re working on this right now.

But that’s why I said, it’s not satisfying.

This is like the function of sleep.

Why do we sleep?

We know sleep is very important to us,

but we still don’t have a satisfying function

of why do we sleep, right?

We have hypotheses.

But the why questions, I think it’s our good friend

and colleague at University of Washington,

Russ Van Gelder, who always says,

when somebody asks why, the best answer is just to say,

I wasn’t consulted at the design phase.


None of us really know why.

No, but the point is, maybe I shouldn’t have said why.

What is the function of sleep?

It’s still very hard to know.

Why would, what is the reason organisms have to go offline

for so long?

You know, people assume it’s for repair.

Assume it’s for learning and memory.

Assume all kind of stuff.

But there is really no clear function for sleeping.

There is no clear function for sleeping.

I mean, if you talk to people, there are hypotheses.

I mean, all we know is that if you don’t sleep,

or your sleep is very fractured, you get messed up.

And you could die even, right?

I mean, it’s really bad if you don’t sleep.

But we don’t know what is the function,

what is that sleep have done to organisms

that couldn’t have done with rest?

What if you just could rest without sleeping,

just sit down and rest?

Well, my lab’s trying to figure out whether or not

these non-sleep deep rest protocols

can compensate for sleep.

I mean, obviously sleep is better,

but many people are not getting the sleep that they need.

But okay, so, and if people are sensing

that Samer and I are about to start talking

over each other and arguing,

that’s always the goal when we talk, right?

Unlike other scientists I interact with,

when Samer and I get together,

it’s considered a successful conversation

if we get into a big fight and then go for a big meal

where I pick the restaurant.

Okay, so let’s talk about food and eating and appetite.

You had yet another,

yes, I greatly admire your success in this way,

yet another incredible discovery

showing that there are direct, excuse me,

effects of light on appetite and feeding behavior.

Maybe you could just summarize those results.

And honestly, that paper is the one

that allowed us to come with the tripartite model

because we were thinking completely wrong about it.

We wanted this experiment,

it’d be fun for your audience to hear

why we started this experiment.

Remember that when we discovered the IPRGCs,

we figured if they are the only relay

to entrain the circadian clock,

then you could kill them and have an animal

opposite to the one that we spoke,

or a human opposite to the one that we spoke about earlier,

where instead of having no pattern vision

and have circadian photoentrainment,

we could produce an animal that have pattern vision,

but no circadian photoentrainment.

So circadian blind.

Circadian blind, but pattern sighted.

And we succeeded in that.

The problem when you have these animals,

which I’ve told you many times already,

is that they don’t adjust to the day-night cycle.

So doing experiments on them become very complicated.

What is their behavior like if you don’t have these cells?

Are they awake and then asleep, awake and then asleep?

They just drift like the humans we’ve talked about.

They think they’re in Las Vegas

with no clocks or watch, right?

They stay up later every night.

They come either, depends on their clock.

If their clock is shorter, they come in earlier.

If their clock is longer, they come in later.

So they’re really messed up.

They really don’t adjust to the,

if they were in the wild,

they’d be eliminated in a second, right?

There is no way they’ll survive.

So me and Diego started talking and we’re like,

what if we use non-light entraining agent?

And what is the strongest non-light entraining agent?


So we thought that the light defective animals

will have more sensitivity to food entrainment

because as you know more than me,

this is an area that you’ve worked really well on.

For vision, if you’re image blind,

your hearing and somatosensory get improved, right?

The lack of vision improves your hearing and sensation.

But we found actually that if you don’t have

the light system, actually you’re feeding,

the food ability to entrain the animal

goes completely to the ground,

completely opposite to what we predicted.

So light viewing and feeding behavior

are interacting in ways that support one another.

And that’s why we came with the tripartite model.

We figured it’s different than sensation of the environment.

When you sense with vision, vision and hearing interact,

but your vision is a real full modality.

You want to see, that’s what vision want to do.

You want to hear, that’s what hearing want to do.

You want to sense, that’s what sensing want to do.

But for the circadian system, light, food,

all these entraining agent,

they somehow have to interact to keep a coherent system.

You don’t just assume if you remove light,

this one is going to be stronger.

No, they need to know each other’s.

The light informs when the animal’s going to eat.

Well, what I like about this so much is that,

you know, in the other, in the world outside of science,

in which I don’t really exist in,

but that I see a lot of this kind of wellness,

you know, stuff with this,

all this mind-body integration stuff.

It’s interesting because people view the body

more as a system, right?

A system of organs that interact,

as opposed to the way that standard science

and medical profession is like,

you work on the liver or your ear, nose and throat,

or heart and lung or brain or-

That’s a great way of thinking.

But the biology is integrated.


So for somebody who’s interested

in affecting their eating behavior,

something that you are familiar with

and that we will talk more about

your experiences of in a moment,

how should they use light

in order to adjust their eating behavior?


So now that I’ve told you about all this interaction

between the different inputs to the circadian clock,

just, you think about it as an engineer,

what would be the best thing?

The best thing is to know

when your food times happen in the day,

when should you get light,

and when is your circadian clock in your system, right?

So if you eat at very specific times of the day,

that’s another signal that is telling your body,

your clock, you’re in a certain time of the day.

So if you’re having lunch at the correct time every day

and you’re getting bright light,

now you have two systems that are informing your clock,

your clock is going to be better.

So regular mealtimes.

Regular mealtimes that fit your circadian clock.

So, and in fact, if you do that,

when I started doing this,

and it helped me lose weight,

is that I’m exposing myself

to the right amount of light-dark cycle.

I’m eating at regular time.

It is amazing.

You will be not hungry, let’s say,

let’s say you eat at noon.

You will not feel any hunger at 11.45,

and then all of a sudden, the hunger jumps.

This is clearly not an energy issue

because it could not be that drastic.

No, the desire to eat is mainly driven by these cues,

these hormone cues that are very exquisitely timed

to sleep-wake cycle, but also to light.


And you know, in the wild,

you could imagine why energy level

through the arcuate nucleus-

I should explain to people what the arcuate is

because I don’t think we’ve done that adequately.

The arcuate nucleus is an area of the hypothalamus

that drives hunger and feeding behavior.

And what we’re talking about is the fact

that it’s taking cues from your viewing of light,

believe it or not, is impacting your level of hunger.

And this is a non-trivial way in which your timing of hunger

and amount of hunger is regulated

by when and how much light you view.

So let me ask you a couple practical-

But can I just, this is really,

before you ask me, sorry, Andrew,

we said we’re going to fight,

but to me is the interesting thing to think about it.

In the wild, when you didn’t have the availability

of food that we have,

the arcuate plays a huge important role

because if you weren’t successful in getting food,

then the arc is going to tell you,

look, you have to take risk and go get food

because your energy level is very low.

And that’s great.

There’s tons of great research about that.

But I think what’s missing is the fact in humans,

we’re not getting to a situation, most of us,

we’re not getting to a situation

where we have low energy levels.

Most of the time, actually, we eat,

not because we want to,

because we really have low energy,

but because we want to eat.

So I think that’s why I feel that the timing

is very important for us

because we always have enough energy level for us to eat.

Well, I mean, I enjoy eating so much

that I’ll eat just for the sensation of chewing.

I mean, I enjoy the taste too,

and I enjoy the social aspects when those are a part of it,

but I literally enjoy the physical act of chewing,


Which explains a lot.

Okay, so how regular are you,

or do you recommend people be about mealtimes?

Because what I’m hearing is that light viewing behavior

is pretty straightforward.

Get a lot of light in the morning and throughout the day,

minimize it in the evening and at night, generally speaking,

for sake of mood and circadian rhythm.

But for sake of regulating timing and quality,

I should also say a food intake

because people clearly make better choices about food intake

when they are anticipating a meal

and they aren’t constantly hungry.

And so the ability to regulate hunger

for particular phases of the circadian cycle

is quite valuable for all people,

not just people trying to lose weight, but all people.

Are we talking about down to the minute?

Like if I-

Absolutely not.

All right, so 12 noon is my normal lunch, let’s say,

plus or minus-

Half an hour.

Okay, yeah.

So eat around between 11.30 and 12.30.

If that’s the time.

And it depends if you also do multiple meals.

Remember, three meals,

that’s a decision that somebody came up with.

I don’t know why.

Nowadays, fewer people are doing that, I think.


Given our friend Sachin Panda’s work.

Right, I mean, so you could have two meals,

you could have very multiple meals

that are distributed across your active time.

I agree with Sachin Panda’s work

that try to avoid eating

when your system is supposed to be relaxing,

when you’re supposed to be at non-active time.

So limit your eating to the active time of your cycle.

And that seemed to be,

and Joe Takahashi is doing some beautiful stuff on this,

that seems to be incredibly important

for aspect of the circadian-

And for health.

And for health.

Yeah, I mean, we’re referring to Sachin Panda’s work.

He wrote a beautiful book called

The Circadian Code.

Maybe, Samer, with some luck, you’ll write a book as well,

meaning the world would be lucky to have that book.

But Sachin’s data really strongly point to the fact

that liver health, brain health,

metabolic factors and endocrine factors

of various systems and organs

all seem to benefit from having a period

of each 24-hour day in which we are not eating anything

and then eating at very regular meal times.

Let’s talk about eating and meal times

and let’s move a little bit away

from the science for the moment,

although we will return to it,

and talk a little bit more about your experience

with eating and meal times.

So you’re looking in good shape lately.

Thank you.

I know you’ve been putting work into it.

We talk a lot and you’ve been exercising

and you’ve been eating well, meaning quality food.

You just came back from Jordan

where I’m assuming the food is amazing.

Yes, the food is amazing.

And honestly, usually I gain a lot of weight in Jordan,

but this time I didn’t gain any weight,

which was really nice, so.

Yeah, when I met you,

you were probably about 100 pounds heavier

than you are now.

Yeah, 275 pounds.

I’m 219 now, it’s crazy.

You had a lot of vigor then

and you have a lot of vigor now,

but I know that you undertook a very specific protocol

in order to lose the weight.

Based on your understanding of the circadian system

and of light and appetite and mood,

maybe you could just tell us a little bit

about what that schedule looks like.

And we realize that this is not a prescriptive

for everybody, but you found what worked for you.

Maybe just describe those changes.

I mean, honestly, I followed my circadian cycle, right?

What we’ve talked about, right?

So I dimmed the light at night.

I slept at regular hours.

I ate my major food in breakfast and lunch

when I’m really active and I’m really hungry.

And at night when I avoid dinner

because my circadian system really shuts off at three.

I’m an early person.

Like you could give me anything I would eat before three.

After three, nothing appeals to me anymore.

My system is shut off.

What time are you going to sleep

and what time are you waking up?

Oh, so in my case, I should have put this out.

I mean, I go to sleep literally at 9 p.m.

I mean, I literally, five minutes after 9 p.m.,

I’m completely out.

And I wake up between 4.30 and 5 a.m.

So if I extend it, I go to 6 a.m.

But very rarely, it depends on how tired I was.

And that, as I recall, was an important set of changes

for you to be able to regulate your food intake.

Absolutely, because then I’m having very big breakfast.

And again, for different people, it’s different.

I have a big breakfast at 7 a.m. maximum.

So I have a big breakfast, coffee and all this stuff.

Then I have some simple snack around 10.

Then I have regular lunch at noon or between noon to one.

Then I have another snack at three.

And the hardest time to regulate the food

is between 12 and three.

This is when I really feel hungry all the time.

This is your equivalent of kind of late evening

for most people.

So for me, it would probably be between seven and 10 p.m.

Exactly, exactly.

And then at night, I’m completely not hungry.

But usually, as you said, the beauty,

the enjoyment of food, like when my wife cooks

some really beautiful Indian food, I eat,

but I’m not hungry.

And I notice if I eat with that, I usually gain weight.

But if I regulate that at night, I also lose weight.

So there is a combination of all these things

that help you adjust the input of food,

the input of light, the input of the clock,

and the drive to hunger.

Yeah, I appreciate you sharing that.

And I want to emphasize that some people

are not hungry early in the day.

They might be late-shifted people,

in which case eating later in the day

will work well for them.

It will work, as long as they don’t eat early

in the morning.

That’s just, you have to work with your schedule,

with your active schedule.

Yeah, you and I have been talking about this offline

for years.

I’m glad we’re finally having this discussion publicly now.

What we’re talking about, really,

is finding your ideal sleep schedule.


And finding your ideal eating schedule.


And understanding how those two things interact.

And you know, the nice thing, as you said,

finding them out is going to help you

to understand how they interact,

because we know from the tripartite model

that they are all interconnected.

And for each person, they’re going to be

interconnected differently.

So for each person, you would, you know,

for me, if I exercise at night,

I’m going to mess up my whole system.

Believe it.

So when do you exercise?

Morning. Morning works great for me.

I mean, it’s amazing.

Morning exercise for me works great.

I tried one time because it was easier for me

to exercise at night before I leave

when the traffic is there from the night,

and I think that messed me up

because I couldn’t sleep well,

and I couldn’t wake up well,

and that led to more changes in my food.

I gained weight again, actually,

believe it or not, even though I was exercising.

So I think this really makes me think

that you have to think of the tripartite model

to see what is the best times

and what is the best interrelation

between the different components,

as you beautifully said,

between your meal times, your light exposure,

and your sleep, that works for you.

Well, thank you for that.

Usually, Samer’s insulting me today.

He’s complimenting me.

I’m going to compliment him right back

by saying this is the first time

that I’ve ever really understood how,

yes, light can control sleep.

Yes, it can control mood.

Yes, it can impact feeding,

but that it’s really about doing the self-exploration

to align those in the way that works best.

And what I’m hearing, tell me if I’m wrong,

but what I’m hearing is that once you understand

what gives you the best sleep-wake cycle,

then you should exercise during the period of time

in which you feel most alert.

And if it works for your schedule,

ideally, you would also eat during the time

in which you feel most alert,

and then stop eating and stop light-viewing behavior

as you head towards sleep.

So the only thing I would say that complicates all of this,

and that’s what makes me sad, is your light exposure.

Mine personally?

Sorry, I’m just kidding.

The people’s light exposure, right?

This is what complicates it,

because you’re not going to be able to figure all this out

if you’re shifting yourself out of your comfort zone

by viewing light at the wrong time of the day.

So let’s say if you were under an idle natural conditions,

you’re a person who would sleep later than me,

let’s say will sleep at midnight and wake up at 8 a.m.

Let’s say you don’t eat anything till noon,

and as you said, you eat late in the evening.

Then this would be perfect for you.

But now see what happens

if now you include the light component.

Now if you push your sleep from midnight to 4 a.m.,

now you’re waking up in the morning,

and you’re actually really not the morning,

you’re waking up, sorry, at noon instead of 8 o’clock,

and the time where you’re not supposed to be hungry,

now you’re going to start eating directly at noon

or something like that, or even delay it.

And now you’re shifting your whole cycle,

and you don’t know if this interaction

between your sleep, feeding,

and the light-dark environment

are still going to be maintained or not.

And that’s the problem that people have.

You know, as I’m hearing this,

what I’m realizing is most of us, probably me included,

are messing up at least one,

two, or three of these components.

But that the probe,

the way to figure out what’s right for oneself

is to start manipulating light exposure.

And I’m going to be honest,

I’m biased because I believe that light

is the strongest time-giver.

And a lot of people disagree.

Some people think feeding is.

I always thought that light was the primary zeitgeber,

the primary-

Yeah, but a lot of people think it’s food.

A lot of people even sometimes mention social interactions.

They read the literature?

I agree with you.

I totally agree with you.

I mean, my understanding is that light

is the most powerful driver

of the things we’re talking about.

That’s why I think we need to regulate this first,

and everything else fits.

And you know, the nice thing is that

your sleep-wake cycle and exercise

tell you really bluntly if you’re doing it right or not.

Tell me more about that.

I’ll tell you more.

I shifted my exercise.

Honestly, things fell apart like never before.

When you moved from exercising early in the day

to late in the day?

Yeah, it completely fell apart for me.

I didn’t enjoy exercise at night.

My pain tolerance for exercise wasn’t as good.

I’m talking with N equals one, and I’m aware of this.

I’ve never tested this empirically.

But at least to me, it really messed up everything.

I started having problems

because my body temperature would go up,

and that will affect my sleep.

I possibly was running in the gym

with a lot of lights,

so maybe the light was a component.

But for me, exercising in the morning,

it’s so much better for me.

But a lot of people can’t even think

of exercising in the morning.

So it depends on when you feel comfortable

in your sleep-wake cycle and your exercise.

I think that tells you if your system

is in synchrony with one another.

That’s really interesting.

You know, we’re good friends.

Our friend Pat Dossett, right, that we both know,

you know, did nine years in the SEAL teams,

and he’s one of these people,

says, you know, he’s happy to go for a run or swim

anytime between 4.30 a.m. and 6.00 a.m.,

and he’ll train in the afternoon too

because, you know, he’s a SEAL team guy

and they’ll do whatever anytime.

That’s part of the phenotype.

But he feels best doing that, right?

I’m a mid, I like to exercise mid-morning,

and I’m happy to skip eating until 12 or 1.


And I like to go to sleep around 11, 30, 12

because I’m a normal human being rather than you

who goes to bed at 9 p.m.

What about Pat, actually?

I’ve never asked him what time.

So Pat’s ideal to sleep time,

I’ve asked him this, would be around 8.30 or 9.

Oh, sounds like Pat.

No, but he has, yes, but he has two young children,

two years old and a newborn,

and so the cycle is disrupted, right?

Yeah, but that’s known, right?

I mean, the effect of childbearing,

and I think we could talk about this,

that’s more complicated, but that’s pretty much, yeah.

Yeah, I mean, I think we need to come up with a new name

for chronotype because chronotype implies

that it’s just about sleep and wake,

being an early bird or a night owl,

and what we’re also talking about is how exercise

and eating match on to those.

And the phase relation between them.


And the phases between different components, as you said.

Because they interact.

Because they interact.

And they don’t have to be in the same phase,

like let’s say my light and food

could be very close to each other’s,

your light and food could be different, right?

The phases don’t have to be, they can be plastic,

so you have to find this for yourself.

You may be a person who eats late at night,

exercises late at night,

or you may be a person who exercise early, eat later,

so it doesn’t, as long as the phase is good,

that’s what you have to find out.

Okay, and if I understand correctly,

when you’re talking about phase relationship,

it means you want to lump exercise, feeding, and light for-

And sleep.

And sleep in a way that as a coherent and total system

makes you feel really good.

Temporally in a great order.


And I could tell you, to me,

is literally getting exposed to sun,

clearly in the morning, clearly at noon,

I go out, I keep my windows in the office completely open,

eating mostly in the early time of the day,

and exercising, and literally at the end part of the day,

I’m literally in a more thoughtful, vegetative state.

Like I really can’t, like after five,

I tell my students,

if you want to tell me anything complicated,

you’re wasting your time.

My brain just doesn’t function.

So even though I only sleep at night,

but I’m really starting to shut off, ramp down,

really, I mean, it’s, you know,

I could send email, talk about brainless stuff,

but my power, my energy to do powerful stuff

really dropped tremendously.

So all my students who know me very well,

they put the meetings with me early in the morning

because they know this is when I’m,

so everything for me, and for me, is very tight,

so it could be different.

It’s very clustered in the morning,

it’s all tied together,

and literally the remaining part seems to be just a,

you know, vegetative state.

Yeah, you and my bulldog, Costello,

who unfortunately passed away recently,

had that in, yeah.

I did not, that’s so sad.

Yeah, Sam and Costello were good friends.

Yeah, sorry to break it to you here.

Yeah, he had a good long life and he went easy,

but he had a circadian clock

that basically would just sleep around 24 hours a day,

minimal activity interspersed every third day or so.

You do have this morning vigor.


And that’s, I think other people

are gonna have more of an afternoon vigor.

Do you think that this can change across the lifespan?

It’s, the rumor is that teenagers

naturally want to sleep in later and stay up later.

Do you think that’s social rhythm

or do you think that that’s actually biological?

Yeah, that’s a tough question.

I mean, it could be both.

One thing that worries me is that it seems

that if anything with age,

this morning rigor gets stronger.

You mean you want people to become more of morning people?

More of morning people.

Why is that?

Where are you?

I think that’s good.

Because for me, I’m already a very shifted morning.

I don’t want to be one of these 7 p.m. to 1 a.m. sleepers

at some point.

Yeah, on the other hand, it’s also kind of nice

because it’s quiet and you can get work done.

Yeah, but honestly, from 4.30 till 7.30

when my wife wake up, it can be very long.

Yes, you achieve a lot, but it’s quiet outside.

So I don’t want to be at 1 a.m.

Let’s put it this way.

You can tell Samer is more social than I am.

That’s right.

That is true.

But we should touch on that actually.

So your wife, she follows a different schedule.


And so the social rhythm is important.

I think what should we do?

How should we conceptualize?

And how should we adjust ourselves

according to the social rhythm?

And I honestly love this hypothesis

that people came up with.

And Pat’s kids reminded me of.

Because kids are really gonna disrupt your sleep-wake cycle,

it seems like there is a chrono attraction

that usually people who attract each others

have actually different sleep-wake schedule.

And the idea being is that this allows them

to take care of their kids throughout the day-night cycle.

And have a peaceful marriage.

And have a peaceful marriage in a way, right?

So, I mean, we didn’t have kids, me and Reggie,

so maybe this is, but it seems like evolutionary,

it makes sense that if you wanna protect your kids,

you don’t want everybody to be morning rigor

and then the kids don’t have,

so you want the distributed across.

I mean, it makes sense.

It’s a reasonable argument.

I’ve heard that one of the reasons

that people think that the clock is not exactly 24 hours,

but it’s 24 hours plus or minus 20 minutes or so,

is because we believe that we evolved in clans

or groups, villages, whatever,

that were about 100 to 200 people.

And in order to have protection

around the early morning hours

when we’re vulnerable to predation,

and in the late night hours

that you would want some individuals of our species

to be naturally more like night owls

and some more like early people.

So your theory of parenting is similar in that way.

The social rhythm is a powerful rhythm though.

Meaning, if I go out and I’m tired,

let’s say I’m tired at like 9.30,

I don’t want to go out,

like I’m in a meeting.

So can I just say something about that?

I think the social rhythm is powerful

at the obvious levels,

like it affects your sleep,

it affects how much you wake up or eat,

but I’m not so sure it’s as powerful

as people think on the clock.

Now, eventually it will mess up the clock

because now if you’re doing a lot of social at night,

getting enough light,

eating at the wrong time of the day,

eventually you’re going to have an effect.

But I don’t think the social interactions themselves

have been shown to affect your clock very strongly

for some reason.

That’s good to know.

Well, for people hearing this,

they’re probably getting the impression

like I’m the night owl

and then Samer is the one that’s in bed at nine

and then wakes up at four.

But having attended many meetings with Samer,

I can tell you that he’s the party animal.

So let’s talk about that.

I mean, let’s talk about the fact

that you’re the partier who’s up until two

dancing at these various meetings,

which I’ve seen.

You’re actually a good dancer, I’m told.

But what should we do when we do stay up very late

for whatever reason?

Could be because we had to take a midnight trip

to the hospital, the unfortunate reason,

or it could be because you’re in the presence of people

that you don’t see very often

and you go out for a really nice night out on the town

and you get to sleep around 2.30 or three in the morning.

How should one get back on schedule?

Do you force yourself to then get up and view light

at the normal time that you would get up and view light

or do you allow yourself to sleep in?

What’s the optimal protocol?

I would allow myself to sleep in

and remember, this is a long-term effect.

This is something that you live with for a long period.

And remember, I told you about the experiments we did

with the mood.

These require two weeks of that light schedule

to cause mood disturbances.

So these don’t happen just in a single day.

So this is the way you justify staying out late

every once in a while?

In the meetings you’ve seen me,

and I’ve done this for five or six days continuously,

but what you didn’t see that when I came back to my home,

it took me two weeks as if I did a jet lag.

So I really do suffer for two weeks

after doing a six crazy night of staying up at night,

drinking at the wrong time of the day.

So it’s not that I’m completely okay with it.

When I go back, everything goes back.

It takes me actually literally two weeks to recover

from the circadian rhythm meeting

that you’ve seen me partying at some point.

Which is kind of ironic, the circadian rhythm meeting.

People are totally disrupting the circadian cycle,

but scientists are human beings too.

So I think if you do it at very little occasions,

I think you should not worry too much

that this will have a lasting impact.

And the good news is that if you readjust your schedule,

you could come back to it.

The problem is when you maintain these wrong schedule

for prolonged time, it becomes chronic,

prolonged periods of time.

That’s when you have the problems

and the accumulation of the problems.

So when you have sleeping problem,

you produce metabolic problem.

When you have metabolic problems,

you produce lack of exercise.

And you could see how things can spiral out very quickly.

And then it would be hard to come back to it.

Well, certainly sleep disruption is both a symptom of

and a cause of almost all mental health disorders.

And certainly the metabolic syndromes

that people are talking about nowadays

and all of this, it all funnels back to light.

This is what’s so remarkable.

And so we have these devices and I use my phone

and I use my computer,

but do you think that the mere dimming of the screen

or not interacting with screens after,

with say 90 minutes or two hours before bedtime,

according to what we’re saying today,

this should have a profound effect on all these.

And I really believe it does.

Again, I think as Pat has did these inventions

where you get a pouch,

where you put your phone in a pouch.

So what Samer is referring to is our friend, Pat,

this former SEAL Team member

who’s also a very impressive person

in the landscape of business and family, et cetera.

Real, a superhuman by any regard

has this habit of taking his phone

and putting it into a sealed pouch in the evening.

So it’s basically-

And in his program,

he sends you actually these seal, you know, patches.

And so that I think is a great idea

because not only it will take away the light from you,

but it also take away the distraction

because you wanna repair and recover and sleep does that.

And if you have your phone dinging all the time

or the light flashing from it,

it’s you’re just not getting enough sleep

and you’re causing yourself major problems.

I’d never asked you this,

but I realized now that I should have long ago,

but I’ll ask you now,

why and how did you get into all this stuff?

Yeah, I mean, honestly, I, first of all,

I wanted to become a, you know,

I wanted to study genetics

and I knew I wanted to do PhD in genetics,

but I only got accepted in one university at the time

and I joined the learning and memory lab.

And I liked learning and memory at the beginning.

I worked on the snails and Aplasia californica

and started looking at learning and memory.

But then the same lab was looking at these daily variation.

I was really struck.

Like you never think about it outside of science.

It really struck me

that organisms can measure day biologically.

That was very shocking to me.

And I just really got attracted

and I wanted to see why does this happen?

What is the effect of different times of day?

And I just stuck with it.

It was mind blowing for me who was in medical school

that I’ve never heard about it before.

You know, it’s really amazing.

Medicine, I think still now,

we are very good at looking at stuff spatially,

but we’re very bad at looking at temporal aspects.

So we always like to see images, static images,

spatial information.

Take an x-ray, measure a temperature,

measure a blood pressure.

Exactly, but we don’t think of temporal.

And, you know, you talk to John Huganash right now

and he’s telling you the importance of chronomedicine

or chronopharmac, whatever the word is.

And it really just getting the drugs

at the right time of the day

is gonna be essential for our health.

Do you think that’s gonna come from using better trackers

like aura rings, whoop straps, these kinds of things?

I love the trackers,

but I think there’s even more exciting discoveries.

Now you could take a single blood sample

and measure many biological components

and figure where you are in the circadian clock,

something that was very hard to do before.

So if you have a marker to know where you are in the clock,

you could actually understand more the effect of everything,

exercise, feeling, light input.

What is the marker?

So there are some papers from, what’s her name,

Phyllis Z. and from Achim Kramer,

where they measure multiple RNAs that are known to tell you

what phase of the clock is,

or multiple proteins or biological reactions.

And depending on a combination of factor,

not a single factor,

you could tell where you are in the circadian clock.

So they could, instead of just measuring temperature

or melatonin, just one measurement,

and melatonin specifically is also complicated

by the fact that melatonin is affected by light.

And temperature, your temperature and sleep

can be easily dissociatable, right?

When you travel across different times on your sleep

at different times in the temperature cycle.

So having multiple components measured

will give you a better determination of your circadian phase

and understanding your circadian phase in humans

will tell you what is the effect of giving certain drugs

at certain times of the circadian phase.

So in the future, this is gonna be studied

at a much higher level when you can determine the phase

in relation to all the other stuff.

It’s striking to me that in all animals, besides humans,

if they deviate too much from the appropriate exposure

to light and light-dark cycle,

they essentially don’t mate and or die and or get killed off.

But in humans, we are able to override that

at least to some extent.

But the ways in which we suffer

appear to be things like obesity, metabolic syndromes,

reproductive syndromes that accompany the other syndromes,

you know, endocrine syndromes

and mood and depressive disorders.

Is there any effort at the level of the nationally

or laboratories that you’re aware of

to try and use light in order to improve mood

and mental health?

I mean, honestly, this is my moonshot.

This is the thing that I think people,

because I say, don’t take a pill, take a photon.

I mean, you take pills, it’s important.

I’m just making it that really,

we have an opportunity right now

with the incredible advances of LED lights,

of changing spectra of light, of regulating intensities.

And just for simple changes,

you could really improve sleep-wake cycle, productivity,

and still you could actually get more done

because as we’ve talked about,

when you have all these messed up,

now you have to sleep more,

but your sleep is fragmented.

It’s not very good.

And you can’t focus when you need to do it.

And you can’t focus when you don’t have alertness

when you need the alertness.

So having all these,

you could allow you to do even more actually

at the end than less.

And that’s the exciting part of it.

One of the questions I get asked most often about

is about ADHD.

You know, I think there’s a lot of self-prescribed

as well as clinically prescribed ADHD.

People are having a tremendously difficult time focusing

and not just because they’re sleepy,

they just can’t seem to anchor their attention.

And there could be multiple reasons for this,

but there are now several clinical trials ongoing

using light to try and anchor people’s attention

and mood and wellbeing for sake of focus.

And I think that while I love this saying that you mentioned,

you know, take a photon, not a pill,

and with due respect to the need for pharmacology

for certain people,

I think most people just haven’t really dialed in

their relationship to light in a way that allows them

to rule out whether or not they need medication.


That’s the best way to put it.

I can’t add to that.

Let’s talk about jet lag,

but not in the context of, okay,

if somebody’s traveling from Europe to Japan

or from the East Coast,

because that varies tremendously, right?

I mean, there’s as many different variations on travel

as there are individuals out there

and with goals and jobs, et cetera.

But rather let’s talk about what are the two or three things

that people can do to adjust their schedule quickly.

Yesterday I called you and said, look,

I know somebody who’s traveling six hours.

I won’t even mention in which direction,

because I don’t want people to anchor to that example.

And you described some very simple tools of viewing light

a little bit earlier than normal

and getting on the local food schedule, et cetera,

that would allow them to shift more quickly.

And the reason I want to have this conversation is yes,

for the travelers and for the shift workers,

but mostly because of the fact

that you’ve proven again and again

that people are disrupted

in their circadian behavior at home.

So what are the, aside from what we’ve already talked about,

how can one adjust quickly to a new schedule?

Like let’s say fall classes are starting,

you start a new job or you have a baby

or a puppy or whatever.

What is the best way to shift the clock quickly?

So it’s very simple as we’ve talked yesterday.

So imagine you’re in the outside

with no environmental, with no industrial light.

If your body thinks you’re in early evening

and you see a bright light, what does this tell you?

Oh, wait, this is not early evening yet.

It’s still early afternoon or late afternoon.

So I have to delay my clock to go back to late afternoon.

So if you get light early in the evening,

it delays your clock.

So what does-

That makes you wanna go to sleep later.

Yes, it delays your clock.

So you’re in New York, right?

People in Italy have an advanced clock

because they are six hours ahead of us.

So if you’re in New York

and you get light early in the evening,

you delay even further from Italy.

So now you’re delaying away from Italy.

Now the same thing happens.

Let’s say you thought dawn came up

and you thought it’s already dawn,

but it was, let’s say three o’clock in the morning

or four o’clock in the morning.

And then you get a bright light

and you say, oh, wait a minute,

dawn is not up yet.

So I should advance my clock.

Or I’m at night, but I’m getting bright light.

So I should run because dawn is already up.

So then later in the night, later in your night,

and actually it just happens that in humans,

you get a temperature in the deer later in the night,

low temperature in your body.

After that, lights start advancing your clock.

So if you want to go to Italy,

instead of getting light early in the evening,

you want to get light after the temperature low.

So you could advance your clock even before you go to Italy

and you’re catching up to the Italians just by using light.

It’s as simple as that.

So you could do it for every region.

You could calculate how much they are advanced of you.

You could know how much these light shifts happen per day.

And you can calculate what you need to do,

very simple math, to adjust either in direction of delaying

if you’re going from New York to California,

you want to delay your clock,

or advancing if you’re going from New York to Italy.

So in order to make that a visual,

and because a lot of people are listening to this,

not looking at it on video,

we will put a zero cost downloadable figure of this

on the website related to this episode.

But I think I can summarize it in language as well.

If I understand correctly, what you’re saying is,

if your typical wake up time is say 7 a.m.,

then your low point in temperature

probably occurs somewhere around 5 a.m.

And if you view light right around then,

it’s going to essentially advance your clock.

Yeah, because then your body thinks,

oh, it’s seven o’clock,

so advance your clock by one to two hours.

But if I were to view light, say at 3 a.m.,

then it would probably delay my clock.

Yeah. Okay.

Yeah, so, and then let’s say I land in a new schedule.

I want to adjust to a new schedule.

Let’s say I didn’t manage to do anything

with my light viewing before I went,

and I didn’t anticipate the trip.

Suddenly, I’m on a new schedule, okay?

I was told that one of the ways to help shift the clock

and to avoid gastrointestinal issues

is to eat on the local schedule,

to start basically behaving like a local,

even though your circadian clock

will take a little bit of time to catch up.

Absolutely, but you have to remember the light, right?

So let’s, now that we explained it very simply,

let’s take a very simple example, right?

New York to Italy, that’s a simple example.

New York time, Italy time, six-hour difference, right?

So let’s say you fly from New York at night.

You reach Italy at eight o’clock in the morning.

What is the time in your New York time?

Although you reach-

Six hours back.

Six hours back. It’s 2 a.m.

It’s 2 a.m.

So when you land Italy,

you want to avoid light like the plague.

Yeah, you could eat,

but you really don’t want to get a light.

Because otherwise it’s gonna delay you.

It’s gonna delay you, it’s gonna send you to California

instead of sending you to Italy.

Right, and so this is such a key point.

If anyone’s confused about this,

we will put some diagrams up.

But what Sam was saying is so crucial.

Just because getting bright light in your eyes

early in the day is really beneficial when you’re at home.

When you travel to a new time zone,

you have to take into account what your body thinks,

what, excuse me, you have to take into account

where your body thinks you are.

And so if you’re looking at the Italian sunrise,

having just flown from New York to Italy,

and you didn’t prepare for that trip

by waking up a little bit earlier in anticipation-

Multiple days, yeah.

And you view light at 6 or 7 a.m. Italian time,

beautiful Italian sunrise,

you are going to delay your clock.

You’re going to basically throw yourself back to California,

but you are in Italy.

You’re gonna throw your biology back to California,

and you are gonna be up in the middle of the Italian night,

and you’re gonna be-



And it took, because I called Samer in desperation.

A few years ago, I traveled to Abu Dhabi,

NYU Abu Dhabi, to give a seminar.

12 hours out of phase, it’s a 12-hour flip.

And I thought I could just muscle it.

I thought, I’ll get up, just view sunlight

when the sun comes up.

And I fell apart mentally and physically.

And Samer came to my rescue.

I called him.

I said, I don’t know what to do.

And he said, go to the gym at the local dawn,

work out, eat, and then view sunlight starting the next day.

And that basically got me onto schedule.

So I used food and exercise to adjust myself

because my light viewing activity

was just completely out of whack.

Yeah, I mean, and we talked about other details,

so you have to calculate it, but you’re absolutely right.

I mean, it’s very important to avoid getting

the wrong light information when you’re trying

to adjust your body, because otherwise,

it shifts to the other side.

Absolutely right.

Well, you are one of these people that has such vigor.

It’s one of these things where,

having known you all these years,

you have a tremendous capacity for work

and for soccer and for arguing, respectful arguing.

And sometimes, you know-

It’s getting worse with the age.

Yeah, well, we could talk about that offline.

But I think a lot of your vigor

and a lot of your ability to work hard and focus

and really do so many things at an impressive level

is because you think about these issues

and you think about when you’re going

to be optimal for focus,

when you’re going to be optimal for exercise,

and the when is the key.

And I think a lot of people live in the landscape

of feeling like there’s something broken inside them

because they can’t focus or they can’t do it.

It’s subconscious, right?

Remember, it’s all subconscious, these effects.

And Anouk, you’re absolutely right.

Now, honestly, joking aside about age,

I really agree with you that I think part of the reason

I’m continuing to be able to do this,

that I really think about it

and I make sure that I keep everything aligned.

And that actually helps me a lot.

Like, I don’t suffer in sleep.

I don’t suffer in waking up.

I never use a timer to wake up.

I mean, people say, aren’t you scared?

Like, you have to give a lecture at eight or 7.30.

Aren’t you scared?

I was like, there is no way I’m going to go beyond that.

It just, even if I try, I can’t sleep beyond 6 a.m.

in my regular times.

It’s just, it’s not going to happen.

By 4.30, my eyes are wide awake and I’m in bed.

It’s just, system is so aligned.

It works.

A lot of times people will say, how come I go to sleep?

I fall asleep fine, but then I wake up at three or four

in the morning and can’t fall back asleep.

Is it possible that those people were supposed to go

to bed at 8 p.m.?

It’s possible.

I mean, it is possible.

It is also possible that sometimes people will wake up

and go back to sleep.

But yeah, I mean, it is possible.

Or it’s possible that their clock is completely misaligned,

that they are getting maybe a nap time at night

when they are supposed, and then they possibly feel

so sleepy in the day.

So all these are possible combinations.

Well, that’s an interesting idea I hadn’t considered.

So what they think is their sleep,

their body is so out of whack with the light-dark cycle

that it’s actually a nap.

Or the weaker part of the sleep.

I mean, you see this when you travel

to different time zones before you adjust.

You go to sleep really well,

but two hours later, you’re fully up.

Two hours.

If you were so tired and this is your regular sleep,

there’s no way you’re gonna wake up in two hours.

So then you feel very sleepy later in the day

or something like that.

So it depends of how your whole system

is aligned to the environment.

That’s a very interesting idea.

I think that’s gonna resonate with a lot of people.

I wake up every morning around three or four,

I generally use the bathroom

and then I fall back asleep very deeply.

It doesn’t seem to disrupt my daytime wakefulness.

And I think a lot of people obsess over that waking up

and worry there’s something wrong.

Provided they can go back to sleep, it’s okay.

If you can go use the bathroom, go back to sleep,

that should not be a problem.

Maybe some people, when they go to use the bathroom,

they use very bright light

and then they get an alerting signal.

So if you, maybe that could be as simple as that,

that affects you.

Maybe when you wake up, you put tons of light

or you start reading your iPad.

So there’s all these combination

that we still don’t know about

that could be affecting their sleep wake rhythms

and their sleep maintenance.

Do you use melatonin?

Do you take melatonin?

I don’t need it, to be honest.

In my case, there is no reason to use it

because I could guarantee you

that by maybe eight o’clock,

my melatonin has already started to go up.

And by the time I sleep, my melatonin is very high

because I don’t use a lot of lights after sunset.

And light inhibits melatonin.

And light really blocks melatonin level.

You hear this myth that the pineal gland calcifies

as we get older.

Is that, do you know anything about that?

I mean, I’ve heard about that,

but I don’t know what does,

I mean, there’s not very clear evidence

that affects the sleep.

I don’t know much about it, to be honest.

The evidence that I’ve seen is that,

yes, there’s some calcification around the pineal

just because of where it sits in the brain.

It’s close to some bony structures.

But I don’t think there’s any evidence

that it has negative effects.

I mean, if you still have,

you could measure melatonin

and that should tell you if it has,

it’s such an easy thing to do.

I think this is more of a internet wellness thing

that got outside the cage.

You’re absolutely right.


It sounds terrible, calcification of the thing, right?

The heart thing, right?

Yeah, exactly.

Let’s talk about seasonality a little bit.

I learned, and I don’t know if this is still true,

but that most suicides occur in April, in the spring.

I think there’s a poem that says April is the cruelest month.

I think it is, the poem begins.

Are there data that suicides are more frequent

at particular times of year?

And if so, is the spring that time of year?

Yeah, I mean, a lot of people talk about this.

And one of the hypothesis is that the winter months

that are very bad for mood

make people not wanting to do anything.

And they get into such deep level of depression

that when the sun comes up,

they get actually the energy to act on their depression,

which sounds really terrible.

And it is terrible.

It’s terrible.

So that’s the idea that the lack of light

throughout the winter

caused them to go into such depression

that they don’t feel like doing anything.

Then when the light comes in with rigor in the spring,

it gives them that, after all the depression they suffered,

gives them that push to take that sad final act, I guess.

What other seasonal effects

have been demonstrated in humans?

Yeah, I mean, I think in humans, it’s not very clear

because we don’t think about seasonality.

But if you start thinking about us,

I think we go through major seasonal changes.

I really do.

I think our eating pattern change across the year.

I could tell you that me thinking about this,

there is a clear changes that happens to me

across the year.

But for animals, this is really essential

because for animals,

they have to time their mating behavior

with when they deliver their progeny

in the most abundant amount of food.

And artificial light is causing major disruption

because if you change the way these animals

are receiving the light information,

they either start mating much earlier or much later

and their numbers dwindle

and they get into the dangers

of really completely getting eliminated or extinct.

Well, human birth rates are definitely going down.

I mean, in the U.S. In some areas, not others.

But are there other effects of seasonality on humans

that we are aware of?

Honestly, you could see it perfectly, I think,

in Scandinavia.

I mean, you could talk to people who live in-

Sure, they get seasonal depression.

Seasonal depression is one,

but actually when you start asking them questions,

they tell you like in the winter,

they barely could wake up,

they barely have the energy.

Before even depression,

even people who don’t get seasonal depression,

they’ll tell you our energy level is lower,

our ability to go to work is not the same.

And in the summer,

most people actually sleep very little.

They tell you we really can,

we feel like we’re manic,

we have all this energy.

And not in a negative way, in a funny way, right?

I mean, but if we want to sleep,

we have to put this curtain.

I think in these situations,

you could really appreciate the seasonality of humans.

I think we kind of destroyed our seasonality

because we don’t get exposed to that much natural light.

We have all this artificial light.

But I think, honestly,

one of the things that is going to happen

if they follow your recommendations

about giving light at the same time,

giving food, giving exercise.

Let’s be clear, those are your recommendations.

No, I’m just, fair attribution.

What I’m saying is that this is going to cause them

to also experience some changes across the season

because now they’re going to see the sun differently.

If you’re going to go out in the morning,

in the summer, you’re going to get a much brighter,

that’s why I don’t like the change in time.

I know people think, oh, because you’re biased.

Because I think-

Wait, wait, wait, wait, sorry, the change in time,

you’re talking about daylight savings?

Daylight savings.

It’s such a bad idea

because it disrupts that rhythm that you’re having.

Because I think your body, if you keep that rhythm,

you will see the whole seasonality.

And I look at it from a different aspect than other people.

It really, and people say I’m biased

because I’m a morning person.

And it may be true, but there’s situation-

Secret conspiracy about morning people.

Yeah, but there is, if you think about it, Andrew,

there is a situation where you’re getting light

perfectly well, and then all of a sudden

they delay it by one hour because,

and then even though it’s the summer,

your body now, if you’re still not adjusting,

think, oh, wait, what happened?

What kind of happened?

Well, I’m glad you’re bringing this up

because I always thought, you know,

what’s the big deal?

One hour, right?

One hour shift, you know, spring forward, fall back.

It’s so hard to adjust to one hour action.

But this goes back to the beginning of our discussion.

It’s not just one hour.


Because it’s one hour across that one day,

but there’s this cumulative effect on the clock

and these three elements of your tripartite model.

Right, the homeostatic sleep

and the light direct effects on mood.

And when it’s so close, it’s sometimes hard

to figure out how to adjust it perfectly

because, you know, we’re already sleep deprived

in our society.

And then you shift it by, you know,

so it just, it all accumulates and it has no benefit.

Well, you work at a major government organization,

National Institute of Mental Health.

Why don’t we get a campaign for-

Honestly, I have no idea.

I mean, it makes no sense.

No, I’m saying, why don’t we go campaign?

Yeah, I would love to.

I mean, it makes no sense

to have the summer light goes up at 9 p.m.

The light goes down where I live in Baltimore at 9 p.m.

And then all of a sudden,

when you really want to see the light longer in the day,

you now shift the other way.

And now it goes all of a sudden at 6 p.m.

Why do you do these drastic changes?

Well, let it blend across the whole season.

Yes, later, earlier at night,

but it’s at least consistent.

It goes in a very consistent manner.

I just don’t understand why they do this.

It makes no sense.

Well, I think that the reason they do it

is because they don’t understand the biology.


Because one hour seems trivial

unless you understand the repercussions

of that one hour shift.

Because what’s also clear now,

based on what you’re saying,

is that that one hour shift

is taking you out of alignment

with the natural light-dark cycle

in exactly the wrong direction.

It’s pushing people to get even later in the summer

when light is going to push you later anyway.

It doesn’t make sense.

You put it beautifully.

I just rambled and this is-

No, no, you made it clear.

I mean, it’s like literally it made you,

it made people who are having problem

having an advanced sleep rhythm

because they are delayed.

Now you give them this hour

to make them even more delayed.

You push them even later in the day-night cycle.

It just doesn’t make sense.

I think 2022 should be the year

that we abolish daylight savings.

That would be the day for me, honestly.

Well, also if it has a positive effect

on what is essentially an epidemic

of mental health issues

and other issues related to improper interactions

with light, that I think is a well worthwhile cause

and we can explore it.

So for once, we’re going to fight with another group.

A common battle as opposed to with one another.

I mean, the circadian people, honestly,

to give them credit,

have been trying for years to abolish daylight saving.

Yeah, the problem is they all go to sleep at 9 p.m.

and wake up at 4 a.m.

so we never see them.

That’s right.

No, the circadian community has done an amazing job

of figuring out what we need.

And then the challenge, of course,

is making sure that people get what they need

and making sure that at a societal level,

we’re not vaulting ourself into the wrong direction.

And the biggest problem is that the late waking people,

they think that really,

and I’m going to try to put it in a better way now,

they think, oh, because you’re a morning person,

you want to see the sun early,

so you want me to suffer it dropping late.

But that’s not the case because what happens

is when they shift it back after the daylight saving,

now they’re going to make you suffer really badly

because now it’s going to be earlier.

In the fall when there’s not enough light,

if they keep it the same way.

So try to convince them that actually this at the end

causes more trouble when you need the light

for your late schedule in the fall when they shift it back.

Then they say, keep it daylight saving all the time.

And that has been proven.

That is very bad.

Like people have done studies

that literally two areas close to each other

and areas that were the whole year on daylight saving

has much more problems,

even in cancer rates and depression.

So you don’t want to do that.

So that’s what trying to convince people

that you need to prevent that switch

and you don’t need daylight saving at all.

That’s where the problem happens.


I had not thought about that,

but yes, you late risers in the fall,

when you fall back, as they say, spring forward,

fall back, you dial back the clock,

it’s really compounding the problem that already exists.

And it’s really nice if you think if you keep it consistent

in the spring, you get the equinox

and then the days start going up.

And then even in the summer start going down

and then the fall, you get the other equinox and go back.

So it’s very symmetrical, right?

It goes into short day, longer,

long, long, long, then short day again.

But now you’re getting these bumps in both sides

of the spring and fall.

Why would you do that?

Something that is beautifully symmetrical,

beautifully smooth, you’re putting bumps into it.

Well, and we, not just beautiful because it’s there,

but evolved.

I mean, essentially this is the system we evolved in

for hundreds of thousands of years.

Even apart from the exact equator,

every part of the earth have seasonality.

I want to briefly touch on something,

which is individual and genetic variation

in sensitivity to light.

So not chronotype,

but first of all, a very basic question.

Do people with light eyes, light colored eyes,

are they more sensitive to light

than people with darker pigmented eyes?

I mean, honestly, it makes sense they will be more

because if you think of my dark pupil,

it’s blocking more light.

So if you have light pupil,

yes, for vision, it may not be very obvious,

but for something that is measuring the amount of light,

you’re getting more light than me.

So you probably need less light

to be effective as somebody who’s darker.

And that maybe could explain

why sometimes lighter people say,

I don’t want to go into very bright conditions

because it’s really bright.

Yeah, I can’t even be at a cafe

with one of these reflective tables, like a metal table,

unless I have very dark sunglasses on.

It’s so bright, it’s painful for me.

Whereas some people like you,

we’ve sat outside and had meals and you’re like, fine.

I assumed it was kind of Jordanian toughness versus, you know.

It’s really the pupil blocks more light.

So I think it is possible that it’s as simple

as the pupil blocking more light can have sensitivity.

But your question is also goes deeper.

Are there more sensitivity differences?

And my understanding would be,

I would think that it may be,

it depends on how effective your cells are

in responding to light, how healthy your IPRGCs are.

So I would, but there’s not many studies to show that.

What is really clear that is happening

is that patients with bipolar,

they seem to have different sensitivities to light.

So it seems that at least people

who have psychological changes,

they may have differences to the sensitivity of light.


Where are those differences in a particular direction?

I don’t remember the exact.

We’ll have to, we can look it up.

Yeah. Yeah.


You know, and people have heard me say this ad nauseum

to the point where they actually roll their eyes,

but you know that these are the only two pieces of brain,

you know, I’m pointing to my eyes, folks,

that are outside the cranial vault.

They are two pieces of brain

that basically inform the brain

about whether or not to be alert or asleep.


But you can imagine that those two little pieces of brain

that we call eyes would have genetic variations.

Of course, eye color is genetically,

modular, is that determined,

that there would be genetic variations

based on whether or not your ancestry evolved

near the equator or further from the equator, right?

I mean, you see more blue eyes in Scandinavia

than you do at the equator.

Absolutely, I mean, it’s the lack of light

that said you need less inhibition

because there’s not enough light, right?

So that’s the idea of the change in color.

So yeah, I totally agree with you.

I mean, I think this is an area that will be studied later

and will be empirically determined.

The problem we have in this field right now,

which I think is the biggest problem,

is we don’t have a way to measure

the IPRGC sensitivities in humans.

So we still, like, it’s easy to measure

your rot cone function if you go to an optometrist,

they measure all the details, right?

Contrast detection.

You look at the chart, you look at the SNLN chart,

you look at the letters of the DMV, yeah.

But for the non-subconscious,

we still don’t have a good measuring systems

to figure out what is Andrew’s sensitivity?

What is Samer’s sensitivity?

What is this person’s sensitivity?

And I think we’re starting to work on something like that

to hopefully develop these techniques.

But till we develop them,

it’s going to be very hard to figure out

if there is a sensitivity difference,

how do they relate on men and women,

you know, dark and light,

and all the, you know, normal versus psychologically

effect and stuff like that.


And every time you talk, I learn so much.

It’s like a, in the best way,

the best sense of the term,

it’s a waterfall of knowledge.

As a final question,

I have a question about sensitivity of a whole other kind,

and that’s the sensitivity to spicy food.

Now, the reason I’m asking this question,

what seemingly out of the blue,

is that I made the mistake once

of having Samer cook for me,

and I said, not too spicy.

And he said, okay, not too spicy.

He actually said, okay, not too spicy.

And it almost killed me.

Like, it was like two or three days.

So you know a lot about biology

outside the visual system, light, et cetera.

You’ve been around a while.

Are there known genetic or inherited

of any kind sensitivities to spicy food,

to things like red peppers and capsaicin?

Because what you call mild,

my friend, almost put me into the hospital.

I think this is similar to you swimming in the ocean,

and I need to develop the pain tolerance.

Okay, true, true.

I like cold water swims, and Samer’s not a fan.

But that’s gonna change.

It’s adaptable.

That’s gonna change.

That’s my belief.

Before I met Reji, I was like you.

And once I started eating a lot of spicy food,

I lost touch of how spicy my food is.

So I nearly killed you, Andrew,

and I apologize for that.

I forgive you.

So basically what you’re saying

is that marriage toughened you up.

Toughened me up, exactly.

Maybe that’s the solution.


Samer, this has been an amazing march

through the importance of light,

not just for regulating sleep and wakefulness,

but also for food timing,

the interactions with mood,

the interactions with exercise.

I’m certain that people are going to start thinking

about how to change their relationship with light

as a way to anchor everything that they do,

and that’s important to their health.

And I just, on behalf of all of them,

and just directly from me as your friend

and as a colleague for many years now,

I just want to say thank you

for the incredible work you’re doing

and for sharing it with us.

Thank you so much.

And actually, now thinking about all of this,

and you said I should write a book,

I should write a book and call it The Tripartite Model.

I think that would put all these components together,

would be very interesting to do at some point.

You should write a book.

You should write.

They’ll probably try and change the title

to like Food, Mood, and You, you know,

or something because,

but you can put in little print

on The Tripartite Model or whatever.

But regardless of what it’s called,

you absolutely should write a book.

And so if you’d like Samer to write a book,

or if you’d like to learn more about him,

let’s talk a little bit about where people can find you.

Your laboratory is

at the National Institutes of Mental Health.

He is head of the Chronobiology Unit,

all these things that I’ve mentioned earlier.

But you are active on Twitter and Instagram.


So what is your Twitter handle?

It’s at Samer Hattar.

And we will provide a link for that in the show notes.

Sorry, yes, at the Twitter, at Samer Hattar.

And I think the same for Instagram.

Yeah, actually.

And Samer has been coaxed onto Instagram.

So he does post from time to time,

mostly pictures of food that is incredibly spicy,

but also information about chronobiology.

He comes on for an Instagram Live

every once in a while with me.

And so definitely give him a follow there and on Twitter.

And I’m sure that he’ll be happy to answer questions

and entertain any and all discussions about chronobiology.

Absolutely, yeah, and light.


Great, thank you, Samer.

Awesome, thank you, Andrew.

Thank you for joining me for my conversation

with Dr. Samer Hattar.

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