Season 3

Episode 6 – Effect of Sunlight and Sleep on our Circadian Rhythm – ft. Prof. Satchin Panda

Episode 5 Episodes list Episode 7

In this episode, Bea talks to Professor Satchin Panda, from the Salk Institute for Biological Studies, about circadian rhythm and its connection to light and sleep.
Satchin gives an introduction to circadian rhythm, a mechanism that allows us to stay at our optimal physical and intellectual health during the day, as well as to repair and rejuvenate in the night. He emphasises the numerous health problems, short- and long-term, that can be caused by disrupting one’s circadian rhythm but also highlights the ways, in which we can take control back.
Satchin talks about the role of light, specifically bright blue light, in resetting the circadian rhythm and the discovery of melanopsin, retina sensor that sends the information about the light to a master clock in our brain. Bea and Satchin also discuss how red light might help us to fall asleep, why the role of melatonin in sleep seems to be overestimated, and the role of “sleep debt” in our lives.

To better understand your circadian rhythm, check out Prof. Panda’s page or download the app “mycircadianclock” You can purchase Prof. Panda’s books “The Circadian Code” and “The Circadian Diabetes Code”.

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Bea: Hello and welcome back to the Offspring Magazine the Podcast! It’s Bea and I will be hosting today’s podcast. Today we will be talking to professor Satchin Panda, a professor at the Salk Institute in San Diego, California, who is a leading expert in the field of circadian rhythm research. Satchin Panda is the author of two books: “The Circadian Code” and “The Circadian Diabetes Code”. In part one of this podcast, we will talk about what is the circadian clock and how can we regulate our circadian rhythm through sunlight and sleep. I hope you will enjoy this episode and stay tuned for part two, which will be released next week, where we will dive into his research on time-restricted eating.

B: Hello, Satchin Panda, thank you so much for joining us today! Why don’t you start by just introducing yourself, telling us what you researched, where you’re based, and kind of how you became interested in the research that you do?

Satchin Panda: Hi! I’m happy to be here, thank you for hosting me! I’m Satchin Panda, I’m a professor at the Salk Institute for Biological Studies located in San Diego, California, the United States. And my primary research interest is in circadian rhythms or circadian clock. In fact, my primary research, my primary curiosity is to understand what makes us healthy and what enables us to perform at our peak performance. But if we think about it, “being healthy” is more than the absence of disease. Rather, being healthy is to be fit and aloft during the day and being able to calm down and sleep at night. For most people, when they feel healthy, that’s what happens. In fact, if we dive deep one more layer to understand what is healthy, it’s obvious that there is an intrinsic program in us to enable us to be at our optimum physical intellectual and emotional health at every hour of the 24 hours day. And also there is an intrinsic program to repair, reset, and rejuvenate every night when you go to sleep. So this concept of being fit around the 24 hours day attracted me to the study of circadian rhythm, which literally means a 24 hours rhythm, so 24 hours clock.

B: Yeah, so I think, actually, a lot of people don’t know what a “circadian clock” is, so maybe let’s start by explaining what a circadian clock is, and what happens when we have a regulated circadian clock or a dysregulated circadian clock?

SP: Yeah, so the circadian clocks are programmed in our body, almost in our DNA, so circadian clocks are the intrinsic programs that determine what time of the day or night which genes turn on and off in different organs or in different cell. So these timely activities of different genes and their products, for example, hormones, digestive juice, brain chemicals, almost everything in our body, this timely rise and fall of these gene products actually keep us healthy by enabling all of our organs to perform best when they’re supposed to be, and also automatically repair, and reset, and rejuvenate during the downtime. So in fact, the circadian clocks are the primordial program in our body to keep us healthy around the 24 hours day. And as you can see, if circadian clocks are out of balance, then you can immediately imagine that, the first thing that you’re most familiar with is, you may not feel a lot during the day. In fact, due to mis-timed circadian rhythm you might feel very sleepy and groggy during the day, you may not perform very well. And conversely you may not be able to sleep and stay awake all night. Although these are the superficial level that you will see, that you’ll experience, at the same time, your organs may not function well, your brain chemicals are out of balance – so your learning and memory, your ability to process information, to act on the information, that will also dampen, deteriorate. Similarly, your liver, kidney, heart, muscle, all these organs: they may not perform at their peak performance level. And if it continues for a day or two, then you might feel just discomfort, not disease. But if it continues for weeks, months, or years, then what we have seen is: people get predisposed to a lot of different diseases – from depression to dementia, or to mild infection, to even cancer. So what we’re finding is, based on animal studies, control clinical studies, and epidemiological studies: chronic circadian rhythm disruption due to a lifestyle, where people stay awake late into the night or eat at the wrong time, leads into to circadian rhythm disruption. When this circadian rhythm disruption happens, then, based on all these studies, what we’re finding is: more than 100 diseases, the risk of which goes up. And as a consequence or as a simple correlation is: people who do night shift, or early morning shift, or evening shift, they constantly go back and forth between their daytime activity and nighttime activity – that disrupts their circadian rhythm. And as a result, they’re also at a very high risk for cardiovascular disease, certain type of cancer, weight gain, diabetes, and more disorders. So what we think is: circadian rhythms or circadian clocks are the primordial program to keep us healthy. And when the breakdown we rapidly progress towards chronic disease.

B: So you kind of touched on this; but how can we actually control our circadian rhythm? So in your book, that I also read, you talk a lot about sunlight, sleep, and eating at the right times. So would you say that those are also the three factors that influence and regulate your circadian rhythm?

SP: Yes, that’s a great point. With almost all animals, plants, and other species on this planet, we all have circadian rhythms to adapt to the very highly predictable day-night cycle that we experience. But at the same time, due to the tilt of our planet and the course of our planet around the Sun, we experience changes in day length between summer and winter time. So that’s why our circadian clocks have been programmed to adapt to this day length changes by responding to change in lighting. So for example, when the sunrise occurs earlier and earlier, our circadian clock has to adjust, and when it occurs later and later, it has to adjust. And with sunlight; with daylight, there is also change in availability of food and nutrition. So, accordingly, our circadian rhythms also respond to change in eating time or what time we eat. So now in modern days, since we don’t actually live according to the sunrise and sunset, we live according to our lifestyle, then miss-timed lighting or mis-timed feeding can confuse our circadian rhythm. But the good news is, we can also take this very fundamental biological principles – about the effect of light and food on our circadian rhythms – by determining or by optimizing what time of the day or night we should eat or fast, and when we should get exposure to bright light, and when we should avoid light. So that’s why, as you said, the factors like sunlight, and when we sleep, and time we should be eating, which we’ll cover in a short while, or when we eat: the these are the best tools you have to control your circadian rhythm and, at the same time, consequently, to control the rise and fall of thousands of genes throughout your body.

B: So actually, a question that I just came up with was so: I also read that every organ in your body has a individual kind of circadian clock, right, but how do all these different organs sense, like, light because they all are in a dark environment? Yeah, I don’t know if you understand that question?

SP: Yeah, so you’re right, almost every cell in our body has its own circadian clock because these cells have to take some timing cue, local timing cue right away, and then the question is: how do they respond to light? Are the cells sensitive to light? Not really, we haven’t seen… for example, many blind people who have lost both eyes, unfortunately, they cannot control their circadian rhythm. And that was the earliest proof that light coming through our eyes, somehow, in trend, or synchronizes our circadian rhythms to the outside world. And, subsequently, people are looking for this light receptor, because there are some blind people who have their eyes intact but they have lost their light receptors, rod and cones, they can actually reshut their clock, and then trend their behavior to the change in season. So that’s why people thought that there might be new light sensors in the eyes, that even some blind people and all normal vision people might have, and that led to the discovery of melanopsin, blue light sensors. These sensors from the retina send the light information to a master clock that’s present at the base of our hypothalamus brain, called the suprachiasmatic nucleus. And this master clock, just like the reference atomic clock that we have, sends this information directly or indirectly to other parts of the brain, which, in turn, tell the rest of our body, what time it is outside, or when the sun comes up, and when you flick the light switch off. So the bottom line is, although every cell has its own clock, every cell doesn’t have its own light sensor. They primarily depend on the light sensed through our retina and sent to our master clock in the brain to sense what time it is outside or whether it’s day or night.

B: Yeah, that makes a lot of sense, thank you. So now, that we’re on the topic of sunlight, let’s talk a bit about how sunlight affects the circadian rhythm. So what is it exactly about the sunlight that controls and regulates our circadian clock? Is it the brightness of the sun?

SP: Yeah, so actually, this goes back to the discovery of melanopsin, because for a long time people knew, based on very controlled physiological experiment, by shining different wavelength of light or different colors of light to see what color of light or spectral quality of light is most effective in resetting our circadian clock. And scientists have found that blue light or cyan-blue color, which peaks around 480 to 500 nanometer – that’s the most effective color of light that resets our circadian rhythm. But then the question is: what is the brightest source, best source of bright blue light? And actually, sunlight or daylight is the best source of blue light. And that’s why we say that the daylight or sunlight is most powerful in resetting our clock. But at the same time, our circadian system has also evolved in a way that it can filter out light noise. For example, when our ancestors, hunter-gatherers or farmers, were living without electricity, electric power, and were dependent on natural light, starlight, firelight, or even moonlight, then the circadian system was optimized to filter out this light because, although in full-moon night, one can see his or her way through a jungle or open field, that should not suppress our sleep, otherwise our people would be sleepless on full-moon night or around full moon. Similarly, on a stormy night, if there is lightning, then that should not to disturb our sleep, we should not stay awake. So as a result, this melanopsin system or the light sensing system that resets our clock filters out dim light, orange light, which comes from fire, and also occasional lightning – if there is just sparks or some occasional bright light flash that does not disturb our circadian clock. So if we combine all of this, then the bottom line is: we need bright light of several minutes or even hours that is ideally enriched in blue light – that will be most effective in resetting our clock. And that’s the reason why we consider that daylight, which is the biggest source of blue light, is most effective in training our circadian clock.

B: Could you buy some blue lights and then wake up to blue light instead of white light? And kind of mimic the daylight in that kind of way?

SP: Well, you can, potentially, and it’s being used in many countries. Then the question is: how much of blue light or how much of daylight or bright light you should use? For example, if you just use the very simple measure of light, which many people are familiar with, that is lux: so, for example, one lux says the brightness of light if you are sitting one foot away from a candle in a dark room, which is very dim. And 1.000 lux is 1.000 times of that. And 1,000 lux is the amount of light that you’ll see if you’re sitting next to a large window in a bright day, there may not be sunlight falling into the room but that’s the kind of light you would experience if you’re sitting next to a large window. And if you’re outside in a cloudy day, then the light level can be 5.000 lux to 10.000 lux. And if you’re outside in a sunny day, then the light level can be somewhere north of 100.000 lux, it can go up to 200.000 – 250.000 lux. So now, how much light is sufficient or enough to reset our clock? Although people may vary, there might be person-to-person differences, scientists overall agree that if you have 1.000 lux of light for 30 minutes to an hour, then that’s sufficient to reset our circadian clock. So as I mentioned, 1.000 lux is how much you experience if you’re sitting next to a window, even if you’re not getting sunlight indoor. And 1.000 lux is also the amount of light you get if you walk into a grocery store or a drugstore at night. Because these days most of the stores are bright blue led light, very rich in blue light, and they produce more than 1.000 lux. So now, if you want to redesign your lighting environment, then it’s better to sleep in a room that’s completely dark: you should have night-out dark curtains and then, if you plan to wake up to a bright environment, then yes, you can have lights that produce 1.000 – 5.000 lux of light – wake up to that light, or even you can have bright light in your bathroom so that when you get up and start your day you’re also exposed to bright light.

B: Yeah, so what about people in Norway, for example, maybe they would benefit from these blue lights in the morning? Since, I mean, I think, especially in the winter, their circadian clock would probably be very disrupted, since they barely see any sunlight?

SP: Yes, in fact in many northern countries, people go to light spa. And they’re very familiar, they’re aware of the the benefit, beneficial effect of light on mood and circadian rhythm. And they are the ones who are leading the lighting revolution to bring bright light, a timed illumination, to home. We should also remember that bright blue light also uplifts our mood. So light, in fact, acts as a antidepressant. And it’s plentiful and free during summer time and, as you said, some people who experience maybe winter blues, mild depression, then bright blue light can be the antidepressant, which is plentiful and they can swap their anti-depressant pill with bright blue light. Of course, with doctors advice.

B: Yeah, yeah. So then, I guess you’ve kind of already answered my question because I wanted to know whether there was actually a benefit of being outside more than, for example, the 30 minutes or one hour that, kind of, is, I guess, recommended. But I think if we’re just talking in terms of circadian clock, then I don’t think there would be that much of an added benefit of staying outside maybe six hours instead of one hour? But probably staying outside brings you other benefits, other than just regulation of the circadian clock?

SP: Yes, I mean being in nature also brings many other benefits. People are showing that interacting with nature also improves your cognition, your brain function, so there are a lot of benefits of being outside during the day time, even if it’s cloudy and you may not feel eager to go outside. But even on a cloudy day there are a lot of benefits to be outside.

B: Okay, yeah, so I guess just for circadian clock, it’s enough to be outside for 30 minutes to one hour? That should regulate your circadian clock, reset it. So we talked a lot about blue lights, what about in the evening? In the evening, you don’t want to be looking at this strong blue light because that will prevent the production of melatonin, right? So actually my question would be whether there’s any kind of studies, because I know that a lot of people recommend maybe looking at red light and having red lights around the house, so I was just curious to know whether there’s any studies that kind of really look at what it is about red light that is more beneficial in the evening as opposed to white light? Is it because of just the lux, so red light is, I guess, more dim and so there’s less lux of light and so that’s why it’s more beneficial? Or is it because of the wavelength?

SP: I think you already told me the answer: it’s because of the wavelength. Because as I mentioned, the melanopsin system is most sensitive to blue light, so if you avoid blue light, then it can reduce the effect of light on circadian clock and also effect of light on melatonin. You mentioned melatonin, melatonin is the nightly hormone that goes up at night, presumed to make us fall asleep. And melatonin level usually rises two to four hours before our habitual bedtime, for many people. And exposure to bright light reduces the production of melatonin. So that’s why it’s beneficial to avoid bright light in the evening. And, as you mentioned, there are quite a few studies showing reducing brightness of light can help reduce the suppressive effect of light. And since we know that blue light is most effective in reducing melatonin level, and that’s even before the discovery of melanopsin, this was known, that makes sense to avoid blue light and, if you have to work, then work under orange or red shifted light. You really don’t have to go and buy red light bulbs and replace all your light but, for example, these days there are spectral tuning light bulbs, where you can change the color of the light. And those can even mimic sunset light, or orange light, or fire light, which is not entirely red light but sunsets are orange and that might also help.

B: Okay. Well now, that we’ve already mentioned melatonin and talked a little about bit about sleep, why don’t we go into talking about sleep and how sleep affects the circadian clock?

SP: Yeah, sleep is actually an output of circadian clock and it’s also under its own homeostasis. So that means if I sleep less one night, then next day during the day-time, I would feel sleepy but, at the same time, if I try to sleep, I may not get six hours of restorative sleep that I could get during night-time. So that means circadian clock somehow suppresses sleep during the day time. So that even if you are sleepy, you cannot sleep continuously for six to seven hours easily. And conversely, if you sleep too much at night time, of course, when you’re sleeping, you’re also avoiding light so you may indirectly modulate circadian rhythm by protecting the circadian clock from the resetting effect of light. So that’s, on a very high level, that’s the relationship between sleep and circadian rhythm.

B: And so we’ve already talked a little bit about melatonin, how melatonin, well, you need melatonin to fall asleep quickly, but then if you have bright light exposure then that will reduce the amount of melatonin that’s produced. But if we take away the bright light exposure, what actually determines how much melatonin will be produced?

SP: Well, people feel that melatonin is absolutely necessary for sleep, which is not true. There are many people walking around, who don’t produce any detectable level of melatonin or very little melatonin, they have perfectly fine sleep. Almost all the laboratory animals, particularly rodents used around the world, almost all, not all, they don’t produce any melatonin but they still do have perfect sleep. So we should take away this misnomer that you need, absolutely need, melatonin to fall asleep, which is not true. Having said that, the the biggest factor that determines how much melatonin you produce is age. Because babies, toddlers, and young adults, teenagers, they produce almost 10 times more melatonin than a 70 or 80 year old produces. And then the other factors, you know, the genes, of course, that are involved in melatonin production, they will determine melatonin production. But at the same time, as I mentioned, there are many animals, there are many humans who don’t produce any melatonin but they still can sleep. So the impact of melatonin on sleep, although we know when we take supplemental melatonin, many people respond to that supplemental melatonin and may fall sleep much faster than without. But still, it’s an enigma how melatonin actually makes us fall asleep. And for those who produce okay level of melatonin, like most of us, and we know that in the evening, if we are exposed to too much of light, then that suppresses melatonin and we may have difficulty falling asleep. But all of the light effect on sleep is not necessarily mediated through melatonin. Because, even in animals that don’t produce melatonin, light can still modulate sleep through other ways: through synaptic way or neuronal connection, through that path, without melatonin. So those are some of the things about melatonin, but at the same time, we know that melatonin is, we have to keep in mind that although in European countries, and in Canada, and many other countries, melatonin is highly regulated, you cannot just buy over-the-counter melatonin, in US and in other countries, melatonin is an over-the-counter medication. And in fact, I think this is one of very few or maybe the only hormone that’s not regulated by FDA, which means that too much or too little melatonin will not do much harm. And so that’s why, it’s kind of a little difficult to put a finger on what is the exact role and necessity of melatonin in human physiology.

B: Okay, but you would probably say that, if someone has difficulties falling asleep and they are trying to regulate their circadian rhythm as best as they can, it would be beneficial for them to supplement?

SP: I won’t go that far without knowing why the person is not falling asleep because this would be promoting melatonin supplement, which I’m not here to do.

B: Yeah, of course. Okay, and so what happens then to melatonin in the morning? Does melatonin take a while to drop or does it drop, basically, before you actually wake up?

SP: Yeah, so it depends on how you wake up. For many people, when a lot of people, they wake up without an alarm clock, and the idea is maybe their melatonin levels begin to drop before they wake up, and when it drops to sufficient low level then people can wake up. But, at the same time, just keep in mind that a lot of people who don’t produce melatonin, a 70 year old may produce negligible amount of melatonin, but still they can perfectly wake up fine without the alarm clock. So we have to really disengage this idea that melatonin drives your sleep, and your sleep depends on melatonin, your waking up depends on melatonin – that’s not yet proven, so let’s not go there. What we know is: for people who produce melatonin, it can take one, two, even four hours for the melatonin to drop to daytime low level after they wake up. And this has some potential consequences for their metabolism and how they digest and process carbohydrate. And we’ll get there in the next few minutes.

B: Yeah, so in general, so we’ve talked about how sleep is really important and how having a healthy circadian clock can regulate your sleep, something that really interests me is how, as you mentioned, young babies they, like, sleep really long but then, as soon as you get into the teenager age, then you start to sleep longer in the morning and go later in the evening, but then as you get to the adult age, then you kind of shift back to waking up a bit earlier and going to sleep a bit earlier. So is this an example where you’re kind of changing your circadian clock as you age? Or why is this the case?

SP: Well, there are two different aspects here because, when you’re saying that teenagers go to bed late, that’s because we don’t know whether they go to bed late because they’re more sensitive to light, their circadian system is more sensitive to light, or the circadian clock is different. There is no proof so far that the circadian clock itself changes with age. The clock is still the same, the phase relationship remains the same, what we suspect is: the reproductive hormones somehow change, might change, the sensitivity of the clock to light. Just keep in mind that our exposure to bright light has happened only in the post-industrial era, and in fact our exposure to a lot of evening bright light has happened only in the last 50 to 100 years, because before that light was very expensive. So if we go back to now, communities where there is no electrical lighting and the question is: do teenagers in those communities also stay awake late into the night and go to bed after midnight? We have not seen any literature that says so. So that’s why we suspect that maybe the teenagers are more susceptible to light and teenage years, particularly 17-18, when the kids are in high school, they have a lot of pressure, and we know that these days most of, the at least in the US, in high school, the deadline to submit your homework is midnight. So there is a social pressure for teenagers to stay awake late into the night. Even in colleges in the US, now the deadline for submitting your homework is midnight. So we got to address that issue first before addressing whether their clock is different. The danger is: if we keep on saying that in teenage years the clock somehow has shifted, then that becomes an excuse for everybody to change, to stick to the current policy that teenagers should be driven to their homework, and nightly activities, and everything till midnight. Which is a very dangerous proposition because that, we can use that excuse, to chronically disrupt circadian rhythm in teenagers and may leave the lasting impact through epigenetic mark on the rest of the life. So more research is needed to figure out whether teenagers actually are most likely to benefit from better light management in the evening, whether their life styles should be optimized by health education, and also at school and colleges, and also changing the deadline when they should be submitting their assignments. There should not be any instruction, for example, after 7 p.m. in the evening. Because they also need time to unwind, relax, and fall asleep.

B: Yeah, yeah. Another really interesting concept in your book was this concept of sleep debt, and how you can, if you don’t sleep enough on one day, you can catch that up on the next day. So do you want to talk a little bit about that because I’d actually never thought of it that way?

SP: Yeah, so the sleep debt is almost, like everybody experiences, many people, they sleep less during the week because they’re trying to finish their assignments and going to bed very late in the evening, late at night. Waking up to go to work before eight o’clock or nine o’clock. And almost everybody experiences that, in the weekend, they’re trying to catch up with their sleep. So they’re not actively catching up, actually there is enough sleep debt building up that every day that you slept less – your body is not going to forget it, it’s actually saved in your brain. So that in the weekend, when you have chance, then your brain is pushing you to sleep. And whatever lost sleep you have experienced in the week, it’s trying to get that sleep back. So that’s the concept of “sleep debt”. But at the same time, we must keep in mind that we don’t get back, we don’t actually pay all the sleep debt. Because, for example, if someone is supposed to sleep for seven hours every night, and for five days he or she has been sleeping only five hours, then there is 10 hours of sleep debt because there are 10 more hours to sleep. So as a result in the weekend, somebody should actually sleep 7+10, 17 hours at least, on Friday and Saturday night. But what happens is, since the circadian clock suppresses sleep, doesn’t allow us to sleep during daytime, we actually don’t catch up with all the sleep debt. So we may catch up with 4 to 5 hours extra sleep but not the 10 hours. So to some extent, we cannot just postpone our sleep and hope that we can get that restorative sleep in the weekend, because our circadian system won’t allow it. So even if you try, many people will wake up maybe by 10-11 or noon, they cannot sleep through the day. So there is this balance between sleep debt, which actually happens, and how our robust circadian rhythm can prevent you from getting back on your sleep.

B: Yeah, so I guess it’s really important to still sleep then whatever hours a night and not think, “oh, I can catch this up later on”. Yeah okay, so that about sleep, that was really interesting.

B: That’s part one. Thank you all so much for listening and stay tuned for part two, which will be released next week, where we will continue our discussion with a deep dive into his research on time-restricted eating and how time-restricted eating can influence our circadian rhythm. If you would like to learn more about professor Satchin Panda you can follow him on Twitter or look at his website, and another great way to learn more about his research is to actually read his books: “The Circadian Code” and “The Circadian Diabetes Code”, which have now been translated into several languages and can be purchased online. If you like our podcasts, make sure to follow us on Twitter, LinkedIn, and on Instagram. Thank you all for listening! Bye!

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