Welcome to Huberman Lab Essentials, where we revisit past episodes for the most potent and actionable science-based tools for mental health, physical health, and performance. 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.

And now, my conversation with Dr. Samer Hattar. Samer, thanks for sitting down with me. My pleasure. 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 maybe you could just wade us into what the relationship is between light and these things like mood and hunger, et cetera.

Sure. 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. So that's your conscious perception of light. But light has a completely different aspect that is independent of conscious vision.

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 DNA 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.

How does that rhythm show up in the tissues of our body? 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. The period length of the sleep rhythm on average 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. 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.

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 aspects of survival for animals to have the anticipation and the adjustment to the solar cycle.

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. 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.

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 taught to only relay rod and cone information from the light environment to the brain.

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 these are cells that connect the eye of the brain that behave like photoreceptors, essentially. I think it's worth mentioning now that people who are pattern vision blind, so people who cannot see 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 in training 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, 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. Absolutely. What is the proper way to interact with light in the first part of the day?

Honestly, I 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. Even when it's cloudy, you're going to get enough intensity to help you adjust your cycle to the day-night cycle.

These are general rules of thumb, but how long do you recommend people go outside? So if you do it daily, I would say 15 minutes. If you don't do it daily, you may want to increase it. You do it more, it doesn't hurt. 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. 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.

Okay. 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 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. 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 recommend. So you might want to compensate with some extra time if you missed a day or two.

I've heard you say before, it's entirely possible to get severely jet-lagged without traveling. Absolutely. 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 cycles 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 6 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. There is this idea of chronotypes that we all each intrinsically have a best rhythm of either being a morning person, you called yourself an early person, or a night owl, or more of a kind of standard, you know, to bed around 10.30, up around 7 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. 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 because human notice that people who 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 starts things usually early or late. That's a hard question. We discriminate against late risers. In a way, we discriminate. 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, 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.

Right. And so you're just going to be late. 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.

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 want to go home, you want to read, you want to talk to your kids, you want to 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. you do keep your home quite dim to dark at night. I am an extreme, but I measured it for myself and I asked Reiji, my wife, if she's okay with it.

She also liked the dimness. Both of us can see well in dim conditions, 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.

You know, use red light that is very dim if you want to 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.

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? It actually makes sense that, you know, if you shine a flashlight in your eye, it's much brighter than if you shine a flashlight on the camera.

direct line. So if you just look on the side, most of the light is going to go this way and you're on the same. And even when I check sometimes, I check it so fast and switch it off so fast. So 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. 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.

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, etc. Bingo. Even if we're sleeping and waking up at the appropriate time. Bingo. 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.

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 is your executive brain one of the most elaborated in humans to see that they receive input from these ancient photoreceptor was stunning to us.

How does that finding inform daily protocols for you or for other people? 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. You'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. Let's talk about food and eating and appetite. You had yet another yes, I greatly admire your success success in this way. Yet another incredible discovery showing that there are direct effects of light on appetite and feeding behavior.

So for somebody who's interested in affecting their eating behavior how should they use light in order to adjust their eating behavior? Right. So now that I've told you about all these interactions 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 in this is clearly not an energy issue because it could not be that drastic.

Right. No the desire to eat is mainly driven by these these cues these hormone cues that are very exquisitely timed to sleep-wake cycle but also to light. Exactly. How regular are you or do you recommend people be about mealtimes? Are we talking about down to the minute? like if I absolutely not.

All right plus or 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. Yeah. 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. 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. 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.

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 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 it's I say don't take a pill take a photon and not 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 can't focus when you don't have alertness when you need the alertness so having all these could allow you to do even more actually at the end than less and that's the exciting part of it. Let's talk about jet lag what are the two or three things that people can do to adjust their schedule quickly 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 you 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 meaning that makes you want to go to sleep later?

yes it delays your clock so later in the night later in your night and actually it just happens that in humans you get a temperature in a deer later in the night low temperature in your body after that lights start advancing your clock 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 7 o'clock so advance your clock by 1 to 2 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 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 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 going to delay it's going to send you to California instead of sending you to Italy what Sam is 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 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 and you view light at 2 a excuse me 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 going to throw your biology back to California and you are going to be up in the middle of the Italian night and you're going to be miserable miserable it's very important to avoid getting the wrong light information when you're trying to adjust your body because otherwise it shifts you to the other side absolutely right you are one of these people that has such vigor I think 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 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 system is so aligned it works a lot a lot 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 8pm yeah I mean it is possible or it's possible that their clock is completely misaligned that they are getting maybe enough time at night when they are supposed and then they possibly feel so sleepy in the day so there all these are possible combinations that's an interesting idea and considered so I mean that 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 in when you travel to different time zone 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 going to wake up in two hours let's talk about seasonality a little bit are there other effects of seasonality on humans that we are aware of honestly you could see it perfectly I think in in Scandinavia I mean you could talk to people who live in sure they get seasonal depression what 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 you 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 recommendation 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 much bright that's why I don't like the change in time I know people think oh because you're biased you because I think wait wait sorry the change time you're talking about daylight savings daylight saving 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 if you think about it there is a situation where you're getting light perfectly well and then all of 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 just one hour but the but this goes back to the beginning of our discussion it's not just one hour right 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 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 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 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's really compounding the problem that already exists Samer this has been an amazing march through the importance of light 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 let's talk a little bit about where people can find you your laboratories at the National Institutes of Mental Health he's head of the chronobiology unit all these things that I've mentioned earlier but you are active on Twitter and Instagram right so what is your Twitter handle it's at Samer Hattar and I think the same for Instagram 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 yeah great thank you Samer awesome thank you Andrew you you