ZOE Science & Nutrition - The science of winter depression with Prof. Deborah Skene – leading chronobiologist

Episode Date: October 30, 2025

Try ZOE’s NEW app and gut health test: ZOE.com Winter is almost here! Temperatures are dropping, clouds are descending, and the clocks have changed.  Many of you might be worrying that the Winter Blues are on their way, but perhaps they don’t need to be? Perhaps we can work with our natural, biological rhythms to fend off this blue phase. Today, we’re joined by Prof. Debra Skene, Section Lead of Chronobiology at the University of Surrey and a global authority on circadian rhythms. With over 190 research publications, she’s spent 25 years studying how light and timing affect sleep, mood, and metabolic health. After listening to this episode, you’ll be armed with solid advice that will help you weather winter with a smile, maintaining good health and good mood until the warmth returns. 00:15 The surprising link between your body clock and major diseases 01:25 Quick-fire round: Body clocks, weight gain, and meal timing 03:25 You have a 'master clock' ticking in your brain 10:05 What is 'gut lag'? 11:40 Why light is the most reliable signal for your body (it's not weather) 14:45 Light's 'non-visual' effects: How light affects your mood and performance 16:30 What studies on blind people reveal about our internal clocks 18:50 The shocking health risks of night shift work 21:05 Why you are 'definitely eating at the wrong time' 22:30 The same meal at midnight gives you higher blood fats 25:20 Why living in a cave for a month is 'pretty healthy' 26:20 Are you a 'lark' or an 'owl'? The biology of chronotypes 29:10 What is 'social jet lag' (and why is it linked to weight gain)? 33:05 How winter light changes your body clock 38:30 The direct link between light and 'winter depression' 39:30 Do light boxes actually work for winter depression? 40:40 The discovery of melanopsin: The specific color of light that controls your clock 43:45 Does blue light from your phone really matter for sleep? 45:05 Why light intensity matters more than blue light 48:15 The cheapest and most effective way to get morning light (even in winter) 50:00 Food sets your 'peripheral clocks', but not your master clock 52:20 Why mealtime consistency is the most important rule 53:30 Is daylight saving time bad for our health? 55:35 Summary: Key takeaways for sleep, mood, and eating Have feedback or a topic you'd like us to cover? Let us know here. Episode transcripts are available⁠ here⁠.

Welcome to Zoe Science and Nutrition, where world-leading scientists explain how their research can improve your health. Winter is coming. Temperatures are dropping, clouds are descending and the clocks are changing. Many of you might be worrying that the winter blues are on their way. But perhaps the icy grip of old man winter isn't inevitable. Perhaps we can work with our natural biological rhythms to fend off this blue face. Well, today we're joined by Professor Deborah Skeen,

section lead of chronobiology at the University of Surrey, and a global authority on circadian rhythms. With over 190 research publications, she spent 25 years studying how light and timing affect our sleep, mood and metabolic health. After listening to this episode, You'll be armed with solid advice that'll help you weather winter with a smile, maintaining good health and good mood until the warmth returns. Deborah, thank you so much for joining me today.

Pleasure. Hello. So we have a tradition here at Zoe where we always start with a quick fire rounder questions from our listeners. And we have some very strict rules. We ask you to say yes or no or if you have to a one-sentence answer. willing to give it a go? Sure. Can messing with your body clock lead to weight gain? Yes. Does when you eat influence your circadian rhythms? Yes, we believe that as well, that I think timing of meals very important. Can the color of light influence our sleep wake cycle? Indeed, yes. Some colors are more effective than others. Can light boxes help people keep their

circadian rhythms on track? I think so as well. What's the most surprising thing that you've learned about circadian rhythms in your 25 years of research? Probably the dim light has an effect. I always thought it was just bright light, daylight having an effect on your body clocks, but even dim light has an effect on the clock. So this is a big topic.

So to kick things off, let's go back right to basics. What are circadian rhythms? and why do they matter for health? It's more what are biological clocks, and we have them all over our body, and it's critical that these clocks are synchronized with our life on Earth. For us, as people living on Earth,

we've got this critical light-dark cycle, you know, that happens every 24 hours because of the Earth's rotation. What we need to do to stay in synchrony with this is that we have these biological clocks that are able to respond to the like-dark cycle. And in unison, together, would give you good health. Can you help me to understand a bit more what a biological clock is that's in me? Well, the first one that was discovered, the sort of master clock, so to speak, is in the hypothalamus,

So within your brain, and it has a direct pathway from that clock in your brain to your eyes. So again, the light, dark cycle can enter your eyes and go down this nerve pathway to this master clock in your hypothalamus. And we know it's a clock because you can literally, not in people, but it can be taken out of animal bodies and put in, into a dish and we can see the electrical activity of this clock oscillating. So it's a self-osolating independent clock that can literally tick in a dish. It actually literally is a clock going tick-tock, tick-tock in my brain right now. Yeah, it has a 24-hour rhythm. So it will have a peak and a trough that takes about 24 hours to complete.

So it is a self-sustaining oscillating. and that's in the brain and that's what we call our master clock or our conductor of all these clocks because more recently we've discovered that they're clocks throughout the body particularly in areas that are metabolize so like we have a clock in the liver clock in the pancreas clock in muscles in skin and in adipose tissue and it's all these clocks all around our body that need to all tick in time together and this synchronized clockwork needs to be synchronized to our outside world. Why do we need to have these body clocks inside us? So the real reason we have clocks is to anticipate our next move.

So they act as timekeepers because they're clocks, but they're telling us, when to wake up, when it's appropriate to sleep, when we should eat, when we should not eat, they will anticipate spring and summer, daytime, nighttime. So with the clocks, our bodies can tell the rest of our body, you know, it's two hours before you should be going to sleep. Could you give me an example maybe of something that happens in my body as a consequence of this clock? Yes, well, I think sleep timing. Because, for example, in the evening, we have a hormone in our body called melatonin that's

naturally produced every night. And this begins before you sleep. It is released into our body and it's preparing our body to get used to the idea that you're going to perhaps and should be sleeping about two hours later. So it anticipates a sleep time. And on the other end, we have cortisol, which is another hormone, and this anticipates wake. So we get the increase in cortisol in the early morning hours before we wake up. The clocks are used to prepare yourself for the events of the day. Help me to understand a bit more the waking up, because in my experience, I tend to just go from being asleep to being awake. So what does that mean that my body is preparing to be awake? Right. Well, obviously when you wake up, you need energy supplies. You need to

have glucose. You need to be able to cope. So cortisol is an important hormone here. It also involves increasing the amount of sugar or glucose in your blood. So you have processes that prepare yourself for this wake up. So you're ready to get up to cope. So there's a bunch of things that are going on inside my body, like half an hour before I wake up. Yeah. I'm not even aware of. I feel like I'm blissfully asleep, but actually my body's already starting to sort of warm myself up to get up and hit the day.

That's absolutely correct, yes. Why do we need to have more than one body clock? Yeah. It sounded like you're saying they're almost everywhere. They're called peripheral clocks in our language because they're outside of the brain, but when these peripheral clocks were first identified, of course, it complicated things because we didn't want to know that it's a lot more complicated than we'd originally thought, because then the question becomes, how do these clocks communicate with each other?

Is it via hormonal signal? Is it via a nerve pathway? Is it via metabolites? So people are still studying that. But I guess it makes sense because let's just take the glucose example. If we need to be prepared with glucose ready for waking up, then somehow our brain needs to be able to talk to the liver and to the pancreas and other adipose tissue, different processes that are involved in glucose balance.

That information needs to go from the brain. to these clocks so that they are ready as well. We don't want our brain waking up and the rest of our body clocks or the rest of our organs still fast asleep in bed. That makes sense. So all these different parts of my body have different things they're doing during the day and they need to know sort of in advance. They can't just be told at the moment. They all sort of need this warming up that you're describing whether it's for wake or sleep. And interestingly, there's not just this one central clock that's sort of running on local clocks for reasons we understand or that's just how it is? No, I think it makes absolute sense now that we know they're all there. It just makes

the study of them a lot more difficult. For example, we study shift workers or people who travel across time zones and you've heard of jet lag because of course we're moving very quickly, too quickly from one time zone to the other. And we have sleep problems while we adapt. But of course, the other thing people spoke about quite a long time ago was gut lag. You know, that your toilet function, so to speak, also delay for a while or advance. You know, they mismatched in your new time zone. And it takes a while for that to also move.

But now we've got the idea that maybe different clocks, adapt at different speeds. So this makes it harder. Maybe your heart clock takes five hours to adapt, whereas your liver clock might take eight hours to adapt. So then we've got more of a mismatch between the different clocks. And why that's important is because we believe that if there is a mismatch between different clocks, this isn't good for your health.

This isn't optimizing function. I mean, anyone who knows anything about clock works and how even normal mechanical clocks work, everything needs to, you know, work together to be the most efficient. Got it. So you could have a situation where your brain is sort of on one time zone, my gut is on another and my heart is on a third, and therefore they're sort of pulling against each other. Whereas I think, if I understand right, you're saying we sort of evolves that are all nicely lined up and they all know, oh, it's five in the morning. Jonathan's going to get up in two hours, start doing this,

or it's 8pm, Jonathan's not going to have any more food. Like, I come to the end and I can do these other things. Yeah. How did you feel after your breakfast this morning? Great, I hope. But if the answer is tired, bloated, or hungry again by noon, you're not alone. At Zoe, we always want to learn more about your nutrition habits. So we recently asked thousands of people,

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To download your free guide right now, go to zoe.com forward slash breakfast guide or click the link in the show notes. That's zoe.com breakfast guide or click the link in the show notes. I understand that light plays a very important role in sort of setting the clock. Could you help us to understand that? Well, it's because we evolved to live on Earth. And the most reliable environmental signal for an organism to respond to would be the light-dark cycle. Let's look at it the other way. It's not weather.

because weather can change. Photo period, meaning the length of the day and the length of the night, is a consistent regular event. And so it would make sense that animals would develop a way of using the signal to track the environment. So it isn't weather and it's not temperature because that's too variable. It's not humidity. So photopiered, you know, is the key way that all organisms, all mammals, insects, plants, algae, clocks first discovered in algae, you know, all respond and are able to use this light-dark cycle

to stay in synchrony with life on earth. So this clock is something really fundamental. You're saying it goes all the way back to algae. Yes, absolutely. It's critical that we have clocks so that the algae knows what to do during the daylight and it knows what to do at night. So you'll have these periods of activity and periods of rest that need to be synchronized to the environment. So it's interesting.

So even before we were animals, just sort of when we were like single cell creatures or something, knowing that it's daylight or not has this big effect, whether you need to photosynthesize or I guess for many, many hundreds of millions of years, whether you need to run away from a dinosaur or whatever. Like this is the profound reason why this is sort of central across all ourselves? It's to optimize performance and to anticipate events. And this is about survival. Let's take rodents.

They are nocturnal animals. Now, if they didn't have a clock telling them what time of day it is and that they should go into their burrows in the day, They might be picked out by, you know, those birds flying around in the day. So it's very important that we all have clocks to know when we are safe to sleep and when we should be active and eat. Could you talk a bit about how light is setting those clocks? Yeah.

Well, this is another surprising thing that I think we all discovered, and that is the that eyes and light isn't just to see. We call these non-visual effects. It's not the perfect word. We haven't found a good word for these effects, but they do not involve vision. And we now know all the pathways and the photoreceptors and the pigments that are involved in this process.

It's all happened in the last 25 years. Very exciting time. What we now know is how light that we see when we're outside or that I'm seeing, looking at you, how it affects my body, how it can affect your mood, how it can affect how alert you feel, your reaction time, your performance. So it has effects that are nothing to do with vision, different pathways. And one of these pathways is the critical one in our field that goes straight, from our eyes directly to this master clock in the hypothalamus.

And this was one of the first discoveries, great discoveries, showing how light would work to send the signal to entrain. We use the word entrainment or synchronizing our clocks to the outside environment. So there is a link directly from my eye to this master clock telling it, Okay, this is the morning or this is the night. Yes. Is it only light that keeps my sort of natural rhythms on track? Well, that's been a big debate.

So to get to that, we were studying blind people, totally blind people. In some case, where both their eyes have been removed. So we now know the pathway from light to the master clock is disrupted, gone. then we're able to answer the question, well, what about all these other non-light cues, non-photic cues? So you might say food or social habits. And what we found was that if you didn't have the light dark cycle, if you were totally blind, even with very strong social cues, because blind people, they rely more on social cues, than visual people, as you can imagine, because they don't see.

So they might have dogs, they certainly had families, they were working, so they had alarm clocks, so they had a lot of cues telling them what time of day it is, but they still continued to be desynchronized from their environment. We call it free running because their body clock goes at its natural periodicity. So we all have these clocks and they all have different speeds. For the blind people, their clock continued on their own speed. And that was even with all these social cues. So the line is critical and we think the most important based on these studies in blind people. So, Deborah, what happens if our lifestyle isn't aligned with this sort of natural body clot, this circadian rhythm?

What happens to our body and our health? We would all, as a feel, say that this isn't good for your health. But our challenge is to try and have more detail. It's easy for me to say any mismatch of the clocks in our body and any mismatch between the clocks and the environment isn't good. But probably the epidemiology evidence from shift workers give us some idea because doing night shift work is a sort of test case for really changing your activities. You delay your sleep wake cycle, you change your eating times, you change when you are active and when you're lying down and everything. So everything is shifted, that's why we call it shift work, and it's not good for your health. All the epidemiology is telling us that doing shift work increases your risk of type 2 diabetes,

cardiovascular disease, cancer. And so this mismatch of your clocks and the environment are somehow getting to a point where you increase your risk of fairly major. diseases. I'm a bit shocked to hear that it could even increase your risk of something like cancer, which I never would have thought could be associated with sort of my body clock. I know sometimes scientists say they see this, but these are like tiny changes that aren't really relevant to normal people. Are these like important differences in my risks of the things you're describing? There are a lot of studies now. In fact, cancer was one of the first diseases to be linked and associated with night shift work. And this was the night shift work study in the US, done by

colleagues in the US and in Austria. And it was surprising, but many, many more studies have shown this increased risk. Could you explain to me in simple terms why working at night for a shift might increase my risk of heart disease or cancer? Right. Okay. The first culprit that was produced when we talked about the risk of shift work was light at night. Because now you're in a factory, or even if you're working a night shift as a nurse, you're awake and you're getting light at night. So there was big hypotheses about maybe it's the light at night. But again, a lot of counter-arguments.

So that was the first thing. The next thing is you're definitely eating at the wrong time. Now, one of the major functions of the biological clocks that we've been, talking about, is to partition your food and energy storage and utilization between the day and night. Now, we are diurnal, so we need to be sure that all our metabolic processes are in tune to process the food that we eat during the day. And of course, when we sleep, our body is tuned to fast and different other processes kick in. So our clock is using this energy for activity after eating and storage at night. It's critical that that is linked to the day and night.

Now, the minute you do shift work, you're in a situation where you're eating at night. And your body, especially on the first night of shift work, your body, it's not the right time to eat. And there have been good studies showing that how you process the food and your post-prandial responses, so your after-meal responses to food are delayed and exaggerated. For example, triglycerides, if you eat the same food at lunchtime and the same food at midnight,

you'll have much higher levels of triglycerides at night in your blood. These are the fats. These are the fats. You don't want this at night compared to the day. I mean, these were very early studies done by my mentor at Surrey where we showed that you definitely metabolize food differently. And if this then is chronic, so this isn't just one night of night shift. You're doing shift work or you're doing some day work and then some night work. And your clocks are mismatched. then, of course, this would compromise your metabolism and compromise your health.

And Deborah, I just want to make sure I've got that. You're saying that one of the biggest purposes of this biological clock is actually to separate the times when you're eating and the times when you're not so that your body is sort of prepared to deal with the influx of everything that comes from the food and then have a period when you're not. When you're fasting. And that therefore if you are eating suddenly, like in the middle of the night, when I'm not expecting to it, actually your body isn't prepared, and that has some quite big negative health impacts? Yes, so we think, and then over time, because this, let's say,

this increase in fats at night, based on the food you eat, if that accumulates over time, this could be one of the explanations. Once we knew the epidemiology showing this increase risk of these disorders, particularly metabolic disorders, cardiovascular problems and cancer, people have, you know, really tried to go, well, what is the contributing factor? And so you get all these diagrams of light at night, eating food at night, mismatch of clocks, et cetera, so that it all then funnels down into maybe this is increasing the risk. But as I say, the precise mechanisms are not known and what's the hierarchy?

of these things. Is it because you've shifted your sleep weight timing? Is that the thing that's the most disruptive for disease? Or is it the fact that you've shifted your feeding fasting, for example? So we don't know that yet. If we know that, then we can start giving advice to shift workers and advice to people who employ shift workers. You're saying the thing that is causing most of the problem. It's not that I might be awake during the night and not seen as much sunlight. It's the fact that my body clock is out of line with how I'm living. If I was going to live in a cave for a month, like I'm not having the same risk because I am sort of shifting between day and nighttime and my body clock moving around. Living in a cave for a month is pretty healthy

because you would be just going to your own time. So you would then be living according to your body clock timing. And this, in theory, is the most healthy thing to do. The problem we have is that we have a social clock that's set on our wrists here. These chronologists don't wear watches because we don't need a social clock to tell us what the time is. We've got our body clocks. But anyway, that social clock is telling you, please wake up, please go to work, please go to school.

And this can be in conflict with our body clock. And it is. And this is where we get this thing of the speeds of people's clocks. Now we get clocks that are slow and we get clocks that are fast. And people who have slow clocks are more evening types or owls, you know, types and people who have fast clocks are more morning types or what we call locks and they all have to fit into this sort of nine to five program that we have and so some people are really living against their biological clock system you know if you're a late type and you're being forced to

wake up at 6 o'clock in the morning to go to work. This is a big jaw and a mismatch that does eventually have some health consequences. I'm a bit of a late night person, not very late, but definitely find it quite easy to stay up, find it quite hard to pull myself out of bed early, even if I'm not on something else. Maybe that's just a sign of sort of moral weakness. Is this real? Yes. This is absolutely. absolutely real. And in fact, people who are late types, or at least people who study late types, it's in the biology as well. So we can measure the melatonin rhythm. And that is delayed or later in late types. So it's not something you can say in a questionnaire. There's actually physical hormones. Your melatonin in a late type would only start much later in the evening. than an early type. So they're markers of a thing that you can't, you know, subjectively argue.

And will I be healthier if I live sort of in line with my early or late type rather than just arbitrarily saying, you know, everyone should go to bed at 10 p.m.? Yeah. When we say everyone should go to bed at a certain time, that isn't a rule that we can adopt when we talk about chronobiology or the study of biological clocks, which is what chronobiology is, because everything depends on the timing of your body clock. So their advice is to, hopefully, if you could,

naturally wake up at the time that is your preferred wake-up time, rather than using an alarm clock. And you'll know that most people spend the weekday, let's talk about most ordinary people, they spend the weekday using alarm clocks to wake up and then on weekends the alarms are switched off and everybody sleeps a little later and it's that difference between your weekday working day and your free day that difference till ronaberg has called social jet lag because it's a sort of jet lag It can be up to one or two hour difference.

More social jet lag has been associated with increased body weight. It affects your performance and mood as well. So there's more and more evidence where people study this mismatch between our working day and our normal day. The bigger the mismatch, the more your risk is, the bigger that difference is. So let's say it's two hours. Let's say I wake up at 7 a.m. during the week. and at the weekends I wake up at nine, I'm at risk of putting on more weight?

You are. That's definitely published. Wow, that's extraordinary. For example, during COVID, that social jet lag seemed to get less because people didn't have to get up, get the school bus or go to school or go to work. And so people would have slept a little later. And so that gap between the working day and the free day was reduced. That's really interesting. I definitely shifted my sleep because suddenly I didn't need to commute. You know, in America, especially they have enormous commute times getting to work and things.

And if you can cut that out and use it to sleep, that would be a good health message. I'd love to switch now to what's going on as we're going into winter. So clearly, you've already told me that this light is critical for setting my central clock. And we all know that the amount of sunlight is now shrinking, particularly those of us who are fairly northern. Could you help me to understand what's going on as we're going into winter and this amount of light is changing? Yeah. The winter summer is a phenomenon of, you know, the Earth's orbit around the sun and the tilt. So, you know, we all understand why seasons exist and the higher your latitude, the bigger the photoperiod differences are.

So it's something that is what we live with and we adapt to that. Our body clocks every day they see light. So they know they are tracking time of year as well as time of day. I can't say that with as much confidence in humans as I can in like seasonal breeders or animals because of course you can so clearly see their seasonality. You know, as some animals change the color of their coats, they get fatter, they are breeding and we know how the photo period, the light dark cycle, we know how that affects and changes all of these coat growth and reproduction, et cetera.

So it's a beautiful story, and we know it. The question that we have, do humans have seasonality? That's one still outstanding question that we all trying to study. Do we have seasonality? Well, we certainly have seasonality in, for example, the incidence of diseases, and no better disease to talk about than seasonal effective disorder or sad, as they've coined it, like winter depression. So psychiatric diseases also have different incidences across the year, even metabolic syndrome,

cardiovascular disease, infectious diseases, hugely peaking, as you'd imagine, in autumn and winter. So we have seasonality in the incidence of diseases. Do we have seasonality in our hormones and in our metabolites and in our response to pathogens, et cetera? So, you know, these are questions that we don't fully know the answer to. As we're going into the winter, what do we know is like happening to our body clocks and how might that affect us? Right. So it's getting darker. At every level of the day, so dawn, noon and dusk, the light levels are lower across the board.

And there's less blue. We know that it's the blue light that is more effective than red light. We believe that the biological clocks have a tendency to get later. So we wake up later. There's less light around. the jury's still out about sleep duration, winter versus summer. You never do these studies on the same people. It's very difficult to do these studies on the same people.

You can imagine we've tried. We design the study. We want someone to come back into the lab or we want to see them at every season or every month. It's a lot on the volunteers. The recruitment's bad people drop out. So normally they just take people in the winter versus people in the summer and they're different people. And then we've got studies happening in Scandinavia, which is 70 degrees north. We've got our studies here in Guildford, 50 degrees north.

So that is changing as well. And the biggest problem is we have so changed our environment. So none of us are really living in true summer. and then true winter, because we have artificial light at night in the winter. You know, we use lights more in the evening because we've got darker evenings. We have central heating. So we're not exposing ourselves to the darkness of winter as we would have done like Eskimos. So we've really changed our modern environment to the point that maybe the inherent

seasonality that we probably have is lost because it's so blocked out by all our things that we change in our environment. How does this reduction in light affect us? Because I think I know that I'm going into the winter and I just feel more depressed about it. I'm not to the extent of having depression, but I definitely feel that. I feel this is pretty common. I suspect most listeners thinking about this and not chid up. And it's not only, I think, about it being colder. It feels like that reduction in light itself is somehow having some effect. Is there anything real there? Very real. No, this is really where we have strong evidence, seasonal effect of disorder. So that's in about one to 10% of the population, but it depends on the

latitude you live at. So, of course, the higher latitudes, Scotland, Aberdeen, the incidents would be greater than in South Africa, where I'm from, because the higher the latitude, the less light that you're having. And that is the sort of what we'd call the clinical condition of SAD. but the sub-syndromal, so just under, there's a lot of people in that area where exactly like you say, you feel lethargic, a bit more depressed, you eat more carbohydrates, and it's directly related to light because when people are given a light box, a box that you can put on a desk, and look at in the morning after you've woken up, this is effective. There's very many studies showing the effectiveness of light therapy for seasonal effective disorder. We know it's light. It's a direct effect of light. And the impact you would expect to see is just to affect your mood, or does it affect

other things? You mentioned before about impact on maybe like your cravings and weight gain and things like this. People who have seasonal effect of disorder haven't been shown to have any problems with their biological clocks. So this mood disorder that they have is related to the lack of light. So this is a direct effect of light. We talked already about how light through the eyes has a direct effect on a lot of brain areas. So likely direct effect on mood areas in the brain to improve mood. And Deborah, I know you've studied a lot, the effects of particular colors of light, and I understand blue light in particular.

Why not does the color of light matter? Ah, well, that was exactly our question 25 years ago. When we knew the brighter the light, the more effect, the longer the light, the more effect. But the color we didn't know. And that relates to what photopigments and photoreceptors in your eye are mediating this effect of light on your body. So we systematically had to expose people to very narrow band colors of light. So if you think of the rainbow, we took little very narrow slits of light and exposed everybody to these colors. And you have to also expose them to different intensities.

So different wavelengths, different intensities. And when it all came out, we found that it was light of blue color, about 460 to 480 nanometers. That's the length of the wavelength, if you look it up. And that's because the photopigment in the eyes that was discovered at the same time we were all, you know, in this light revolution, so to speak, It responds to the photopigment of the retinal ganglion cell photopigment, which we've now called melanopsin. You all know about the rods and the cones. That's for vision. But we have deep in the retina another photoreceptor that is directly photosensitive to light, right there, nothing to do with the rods and cones.

and in there contains a photopigment called melanopsin. And the maximum wavelength that stimulates this photoreceptor to have an effect is blue light, 480 nanometers. So the blue light is a thing that's going to really reset my body clock. It is better than green or red. Don't make the mistake that green and red aren't effective. just when you head to head them at the same concentration. Got it.

So everything will reset my body clock, but the blue light is going to have more impact than the other light. So if you want to optimize or maximize affecting your biology, then you design a light that's got more blue in it. But contrary, if you want a light, say in the evening in your bedroom, that isn't going to affect your biology, then you should remove the blue from the light. I actually think that's a brilliant point

to maybe transition to practical advice for listeners. I think you've sort of painted this picture for why this is also important. I'd love to now discuss, okay, what can a listener do to try and have the best possible health benefits? The biggest question I had before this episode was, does blue light from our phones really matter

for affecting my sleep? Yes, but it would depend on how far away, you know, it's all about how much light, how many photons of light get to your eyes. So once you know the principles, you can make it all safer. But of course it depends how close the light is to your eye. The further away it is, the impact is reduced. Then, of course, because blue light is the most effective, as we've described why now, you could filter out your blue so you're well aware of the night shift mode on mobile phones where it gets a bit dimmer a bit more orangey you also get the sort of software that you can put

on your laptops to cut out the blue and so these are all a direct result of our original research showing the blue light is the most effective. All these like gadgets and blue filtering ideas. And so Deborah, I've always thought maybe this was a bit of a gimmick and that the amount of blue light from a phone might not really matter. Was I wrong? The blue part, you know, is the sexy bit, but it's also light intensity. So you do just as well to reduce the light intensity as you would to get rid of the blue. Got it. So it's not like, hey, get rid of the blue. And you're fine. Smash orange in your face, you're fine.

It's more like the biggest thing is just reducing the overall intensity. Yes. So if I am going to be looking at these devices, and I guess a TV would be the same there for as a phone. Just have your television. So the equation for the impact is, it's squared. So really, by every distance you move, you really are reducing the impact by moving it further away from your eyes.

So reducing intensity is key. I mean, now you can buy. blue light filtering glasses that you can wear while you're doing your laptop work in the evening to reduce the blue, and they could also reduce the intensity. So it is about getting less light into your eyes at night. If you know someone whose mental or physical health is affected during the winter months, why not share this episode with them right now? I'm sure they'll learn something that will help them weather the storm. What would your other key bits of advice be for someone who wants to make sure they're tackling

sleep, having just heard you, describe how important this regular body clock is? We're saying don't have bright blue light at night. Please don't. Have dark bedrooms as dark as possible. But the other aspect is the beneficial effects of blue in red light. And that would be in the morning. So to synchronize, we've already discussed how important this role of the light, dark cycle is, is in synchronizing your body clocks. Your best way of staying on track with the light dark cycle is to have blue enriched light in the morning. Now I'm calling it blue enriched light because we don't go around having blue light because that then when you look away from blue light you get this yellow reverb you know it's blue enriched light and LEDs are blue enriched

if you look at the spectral composition of an LED light or the spectral composition of a fluorescent light there's a blue peak in those lights so So that works well for using light, if you have to, in the morning to keep your clocks synchronized to your environment. And so presumably if I live somewhere really nice, like the Mediterranean, I would just go out in the morning and the sun is shining and I would get all of this naturally or if I lived in Africa where we all evolved, this would just happen. Yeah. But for those of us listening to this, you know, in the northern hemisphere and the alarm's gone off and either it's already dark out, still dark outside, or actually it's so grey that doesn't feel very different. What should I do? Well, people use artificial lights that you can buy. But I do want to come back to the cheaper and more effective option is to go outside. Daylight is 500 times more intense than artificial electric light. If you can go out, the best advice would be once you've woken up, naturally, without an alarm clock,

try and go outside within one or two hours and get... And even in the winter, I'm actually getting more light than I... So you're sort of saying it's actually a lot brighter than I realize in my eyes. It's a lot brighter. It's a logarithmic scale. So it's 50,000 lux, 100,000 lux outside. And we're only dealing with 10,000 lux in one of these. Even in England in November?

Oh, yeah, yeah, yeah. And is this only going to improve my mood, or do you believe, I understand you might not have all the data, but do you believe that if I'm doing this regularly, this is supportive for my long-term health? Definitely mood, definitely helping with sleep. but also likely affecting metabolism because clocks are deeply involved in metabolic processes. This is one of the more recent functions of clocks.

I think there's such a brilliant transition to my next biggest set of questions, which were all around meal timing, which is a topic that we've talked about quite a few times on the podcast because there's a lot of nutrition science interest. But you're coming at this from this completely opposite angle. about clocks and I understand you've recently been studying this so what is the advice you can give us everybody has focused for so long on what you should eat but it's now about when and is there a correct or appropriate timing of food and there are bigger experts than me at this but we've done some very controlled lab studies where the question has been okay let's shift the whole meal schedule

by five hours. So that would be breakfast, lunch, and dinner, moved five hours. And then what happens to our body clocks? And food does not affect our master clock in the brain. So we know that now. We were the first to show this. It had been shown in animals, but we were pleased to see this exactly right. So we suspect that food affects probably all.

all the peripheral clocks involved in metabolism, like the liver clock, the pancreas clock, but it doesn't go to this clock in the hypothalamus. So that makes some sense. We shifted this meal timing by five hours, and we found that glucose, the rhythm of glucose, also shifted by five hours. So it has the ability to shift the rhythm of glucose, but not any hormones that are driven by. It doesn't shift the melatonin rhythm, for example. Where it's light shifts the melatonin rhythm, the food only shifts the glucose rhythm.

So you're saying that if I regularly eat on a different pattern, my body, my gut, everything will get used to it and sort of digest it easily if I'm doing that all the time. And so it's almost on a separate body clock. I'm thinking about Spain where they all eat at midnight or something. You're on a schedule where that is normal for you, whereas if I eat at midnight, you know, my blood sugar would go through the. roof. We believe that our bodies have a memory of the meals we've had before. And so, indeed, the healthy thing is to stick with what you do normally. So Spanish people eating late, that's something they're doing all the time. It's any disruption to that normality that is a disturbance for the clock system. So, Deborah, are you saying that in terms of meal

timing and health, consistent timing of meals is important? I think it's going to turn out that it's right, that we should have consistent meals at the same time, and we should reduce the food irregularity. I mean, I definitely know their researchers, you know, at Kings that study this and others. So it's this food irregularity that we are all beginning to think isn't good for your body. I think you're saying that eating sort of at the same times in the same pattern is probably good for your health. But I think you're also saying that if during the week you're waking up early and then having your breakfast at seven, but you're exhausted at the weekend and therefore you're sleeping into nine, you probably don't want to drag yourself out of bed at seven in order to eat your breakfast. Exactly. That's what I'm saying. So when we say to people they should have consistent meal timings that shouldn't change, we should see what those are, you know, first.

I know you've also expressed opinions around daylight saving time, this time when like twice a year our clocks change by an hour. And I was thinking about this this morning because I know it always creates havoc with my kids and particularly my daughter's sort of sleep pattern. It takes at least a week for her to like get back into the right pattern. And I understand we did that because we think it's a good idea when we introduce it as governments, you know, probably a hundred years ago now. is it a good idea?

We believe being on what we would call standard time, which is the time that we have during the winter months, is the better time for your health. Because in the standard time, our body clocks are more aligned with where the sun actually is. So when we're in the standard time, when we go outside at noon and they're more closer together. When we're in British summertime, there's a bigger gap.

So there's more of a mismatch between our body clocks, which is driven by the sun clock, you know, our light, dark cycle, all the things we've talked about today. And this man-made social clock. and any mismatch between our social clock, that clock, and our body clocks, the more unhealthy it is. So how would you study really the effect of, what is it, six months? It's a six-month period. And how do we know how it would be different if we just stayed on standard time? So we can only just use the principles that, you know, we don't want to suggest anything that makes the gap.

between living according to the light dark cycle and the sun clock and our body clocks, and anything that makes that gap bigger with the social clock is got to be worse for our health. That's where we are. Now we just have to prove it. Amazing. I would like to do a quick summary, if that's right, and just correct me if I've got any of this wrong. So I start with my most fascinating discovery, which is chronobiologists do not wear watches. And that's because basically living in line with our own internal sense of our body clock is the best way to be.

And you gave this brilliant example about how daylight savings time actually isn't good for you because you want to be just close to where the sun really is. I think the other extraordinary thing I heard is that night shift work actually increases your risk of cancer, you know, which it's not just, oh, I don't feel very good or I put on a few pounds away. like cancer, diabetes, cardiovascular disease, and it just tells you how important living in line with our biological clock is. Also that everyone should not go to bed at the same time. Some people have a slow clock, which is described as an owl. They want to go to bed late and wake up later. Some people have fast clocks, and so they're morning people, and this is real, it's not made up. There's this thing called social jet lag. So the difference between when you wake up, sort of when you're working and when you wake up at the weekend for most of us matters. And if you said if I had a

two hour difference between this, which I definitely do, that probably puts me at risk of putting on more body weight and reducing my mood. So like that is not a good way to live. We understand a lot about how this all works now. So we understand that the light into our eye is then sending this information to this master clock in our brain that communicates it elsewhere. And this is really important and interesting is that one of the most important things is sort of separating when we eat from when we don't eat. And I think since most of our listeners are very interesting in nutrition, you know, it's nice to hear how important that is. And then we talked about, okay, we're going into the winter. What do we need to do? And I think my key takeaway is

we need to make sure we're giving enough light. And that's at both ends of the day. So in the morning, you're saying actually by far the most important thing you can do is to go outside. There's actually much more light out there than you realize. So if you're not going outside and it's very easy not to do that when we're commuting or we're working from home or whatever it is, that daylight is actually a hundred times more intense than artificial light. And so even though it's the winter and it feels overcast, actually I'm getting a really good benefit. And that's getting me going in the morning. And on the other hand, I really need to think about my experience. At the end of the day, if I want to keep a good cycle of sleep in the winter, the blue light is the most of the most of

effective thing for setting our body clocks or messing up our body clock. And so this idea about putting my phone into like a night mode can make sense to reduce it, but actually don't really obsess about the blue light. It's the brightness that's the most important thing. So if I had really bright light but it's got less blue is actually worse than the normal thing and less. So I should be conscious of that. Make sure the bedroom is really dark. Set myself up so I'm not watching all these bright things before I go to sleep, but recognize that I need more light, particularly if I'm one of these people whose mood has been affected. And it sounds like you're saying everybody's mood has been affected a bit.

Some people have this in a very bad way. And then finally, we talked about, you know, when to eat, which again is going to be even more important, I guess, as we're going through these winter months. And the critical thing here is all this evidence about consistency. So your body is learning these patterns of when you have breakfast and when you have your snacks and when you have dinner and you want to keep that steady. And so again, you'd like to have less of this sort of social jet lag between the weekday and the weekend.

But if you're normally waking up and having breakfast at seven in order to go off to work and you get to the weekend and you're exhausted, don't wake up just in order to have breakfast at seven. You're saying on balance, your gut senses, better to sleep in and let that break. But the more that you don't then stay up really late and try and keep this in sync better for a long-term health. Yes. Well done. That's a great summary. Please come and be one of my students.