The science of winter depression with Prof. Debra Skene – leading chronobiologist

Summary

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ZOE Science & Nutrition: The Science of Winter Depression

Guest: Professor Debra Skene, Chronobiologist
Host: Jonathan Wolf
Date: October 30, 2025

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Episode Overview

This episode explores the science behind winter depression and the critical role of circadian rhythms in health, mood, and metabolism. Host Jonathan Wolf interviews Professor Debra Skene, a leading authority on chronobiology, about how light, timing, and our internal clocks affect everything from sleep to weight, and what practical steps listeners can take to thrive through the winter months.

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Key Discussion Points & Insights

1. What Are Circadian Rhythms?

• Definition & Evolution:

  • Biological clocks exist throughout the body and synchronize us to the 24-hour light-dark cycle of Earth ([02:31]).
  • The master clock is located in the hypothalamus of the brain, receiving direct signals from the eyes ([03:18]).
  • Every organ (liver, pancreas, skin, etc.) has its own peripheral clock, ensuring the whole body’s processes are coordinated ([04:12]).

• Purpose:

  • Clocks help anticipate and prepare the body for activities like sleep, wakefulness, and eating ([05:14]).
  • Hormones like melatonin (prepares for sleep) and cortisol (prepares for waking) are regulated by these clocks ([05:51], [06:54]).

Quote:

"The real reason we have clocks is to anticipate our next move. They're telling us when to wake up, when it’s appropriate to sleep, when we should eat, when we should not eat." – Prof. Skene [05:14]

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2. Why Synchronization Matters

• Clock Alignment:

  • All body clocks must be in harmony for optimal health—mismatches (e.g., from shift work or jet lag) create fatigue and other health issues ([09:26], [10:31]).

• Risks of Desynchronization:

  • Research in shift workers shows greater risk of type 2 diabetes, cardiovascular disease, and even cancer due to out-of-sync biological clocks ([19:02], [20:18]).
  • "Social jet lag" (difference between weekday and weekend sleep) is linked to increased body weight and poor mood ([28:47], [30:17]).

Quote:

"Any mismatch of the clocks in our body and the environment isn’t good. Shift work increases your risk of type 2 diabetes, cardiovascular disease, cancer." – Prof. Skene [19:02]

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3. Critical Role of Light in Setting the Clock

• Light as the Master Signal:

  • Light, especially the blue component, is the most powerful synchronizer of our master clock ([12:52], [14:44], [16:52]).
  • Non-visual pathways from the eyes affect mood, alertness, and performance ([15:24]).
  • Blind individuals lack this light input and cannot synchronize to social cues alone ([17:08]).

• Blue Light Sensitivity:

  • Blue light (460-480nm) is most effective at influencing our internal clocks due to its action on the melanopsin photopigment ([38:57], [40:54]).

Quote:

"If you want to optimize or maximize affecting your biology, then you design a light that’s got more blue in it." – Prof. Skene [41:15]

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4. Winter Seasonality: The Science of Winter Depression

• Seasonal Changes:

  • Light levels are lower in winter, especially at higher latitudes ([31:39], [34:04]).
  • Humans show seasonality in disease incidence (including seasonal affective disorder – SAD), with mood, energy, and cravings affected by reduced light ([33:02], [36:41]).

• SAD & Subsyndromal Symptoms:

  • 1–10% of the population suffers from SAD; many more report mild winter blues (lethargy, low mood, carb cravings) from low light exposure ([36:41]).
  • Light therapy is effective for SAD and mild symptoms ([37:14]).

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5. Practical Advice for Navigating Winter & Circadian Health

a) Maximize Light Exposure in the Morning

• Go outside within 1–2 hours of waking, even in winter—daylight is hundreds of times stronger than indoor light ([46:34], [47:07]).
• Blue-enriched LED or fluorescent lights can supplement, but don’t replace daylight.

Quote:

“The best advice would be once you've woken up naturally... try and go outside within one or two hours and get [daylight].” – Prof. Skene [47:07]

b) Minimize Light Exposure at Night

• Reduce exposure to bright and blue light before bed: dim your screens, use night mode, increase distance from eyes ([42:03], [43:15], [43:41]).
• Blue light matters, but overall intensity is just as important.

Quote:

"It's not like, hey, get rid of the blue and you're fine. The biggest thing is just reducing the overall intensity." – Jonathan [43:28]

c) Keep a Regular Sleep & Meal Schedule

• Respect your personal chronotype: morning lark or night owl—don’t force unnatural sleep/wake times ([27:57], [28:47]).
• Limit the gap between weekday and weekend sleep; minimize "social jet lag" for better mood and lower weight risk ([30:17]).
• Maintain consistent meal timing; irregularity disrupts peripheral clocks and metabolism ([50:46], [51:09]).

Quote:

“The healthy thing is to stick with what you do normally... it's any disruption to that normality that is a disturbance for the clock system.” – Prof. Skene [50:15]

d) Daylight Saving Time:

• Staying on standard time is healthier; daylight saving increases mismatch between clocks and sun, increasing health risks ([52:15]).

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Notable Quotes & Memorable Moments

• On Clocks in Everyday Life:
  “Chronobiologists 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.” – Prof. Skene [25:57]

• On Individual Differences:
  "People who are late types... it's in the biology as well. Your melatonin in a late type would only start much later in the evening than an early type." – Prof. Skene [27:57]

• On Seasonal Mood Changes:
  “People who have seasonal affective disorder haven’t been shown to have any problems with their biological clocks. So this mood disorder... is a direct effect of light.” – Prof. Skene [38:16]

• On Light Therapy for SAD:
  "When people are given a light box... this is effective. There’s very many studies showing the effectiveness of light therapy for seasonal affective disorder. We know it’s light. It’s a direct effect.” – Prof. Skene [37:14]

• Meal Timing Advice:
  “Consistent timing of meals is important... food irregularity isn't good for your body." – Prof. Skene [50:46]

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Timestamps of Important Segments

• Quickfire listener Q&A: [01:32] – [02:22]
• What are circadian rhythms? [02:31] – [06:44]
• Master clock & peripheral clocks explained: [03:18], [04:12], [07:47]
• Shift work, health risks & desynchronization: [18:52] – [25:32]
• Chronotypes (“larks” & “owls”)—biology and health implications: [27:41] – [28:47]
• Social jet lag & its health consequences: [28:47] – [31:13]
• The winter effect: seasonal changes & SAD: [31:13] – [38:16]
• Color of light & blue light’s role in mood/clock: [38:57] – [41:38]
• Practical light advice (phones, TVs, blue light, brightness): [42:03] – [44:19]
• Morning light, going outside, and mood/metabolism: [44:42] – [47:40]
• Meal timing & body clocks: [48:00] – [51:33]
• Daylight Saving Time, standard time & health implications: [51:45] – [53:54]
• Final episode summary by Jonathan: [53:54] – [58:02]

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Take-Home Messages

• Light exposure anchors your circadian rhythms—get plenty in the morning, especially in winter.
• Limit bright and blue-enriched light in the evenings to protect sleep quality.
• Remain consistent with your sleep and meal times, respecting your personal sleep-wake preference (“chronotype”).
• Consideration of daylight saving time and minimizing “social jet lag” improves long-term mood, weight, and general health.
• Light therapy and increased daylight exposure can alleviate winter blues and support metabolic and psychological well-being.

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“Well done. That’s a great summary. Please come and be one of my students.”
– Prof. Debra Skene [58:02]

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Transcript

[Jonathan]

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 phase. 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.

[Professor Deborah Skeen]

Pleasure. Hello.

[Jonathan]

So we have a tradition here at Zoe where we always start with a quick fire round of 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. You willing to give it a go?

[Professor Deborah Skeen]

Sure.

[Jonathan]

Can messing with your body clock lead to weight gain?

[Professor Deborah Skeen]

Yes.

[Jonathan]

Does when you eat influence your circadian rhythms?

[Professor Deborah Skeen]

Yes, we believe that as well that I think timing of meals very important.

[Jonathan]

Can the colour of light influence our sleep wake cycle?

[Professor Deborah Skeen]

Indeed, yes. Some colours are more effective than others.

[Jonathan]

Can light boxes help people keep their circadian rhythms on track?

[Professor Deborah Skeen]

I think so as well.

[Jonathan]

What's the most surprising thing that you've learned about circadian rhythms in your 25 years of research?

[Professor Deborah Skeen]

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.

[Jonathan]

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?

[Professor Deborah Skeen]

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.

[Jonathan]

Can you help me to understand a bit more what a biological clock is that's in me.

[Professor Deborah Skeen]

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 into a dish. And we can see the electrical activity of this clock oscillating. So it's a self oscillating, independent clock that can literally tick in a dish.

[Jonathan]

It actually like literally is a clock going tick, tock, tick, tock in my brain right now.

[Professor Deborah Skeen]

Yeah, it has a 24 hour rhythm, so it'll have a peak and a trough that takes about 24 hours to complete. So it is a self sustaining oscillator and that's in the brain and that's what we call our master clock or our conductor of all all these clocks. Because more recently we've discovered that they're clocks throughout the body, particularly in areas that 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.

[Jonathan]

Why do we need to have these body clocks inside us?

[Professor Deborah Skeen]

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.

[Jonathan]

And could you give me an example maybe of something that happens in my body as a consequence of this clock?

[Professor Deborah Skeen]

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 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.

[Jonathan]

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?

[Professor Deborah Skeen]

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.

[Jonathan]

So there's a bunch of things that are going on inside my body. Like half an hour before I wake up, 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.

[Professor Deborah Skeen]

That's absolutely correct, yes.

[Jonathan]

Why do we need to have more than one body clock? Yeah, it sounded like you're saying they're almost everywhere.

[Professor Deborah Skeen]

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.

[Jonathan]

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.

[Professor Deborah Skeen]

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're 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.

[Jonathan]

And.

[Professor Deborah Skeen]

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 clockworks and how even normal mechanical clocks work, everything needs to, you know, work together to be the most efficient.

[Jonathan]

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're sort of evolved. So they're all nicely lined up, and they all know, oh, it's. It's 5 in the morning. Jonathan's gonna get up in two hours, start doing this, or it's 8:00pm Jonathan's not gonna have any more food. Like, I come to the end, and I can do these other things.

[Professor Deborah Skeen]

Yeah.

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[Jonathan]

I understand that light plays a very important role in sort of setting the clock. Could you help us to understand that?

[Professor Deborah Skeen]

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 photoperiod, 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 photoperiod, 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.

[Jonathan]

So this clock is something really fundamental. You're saying it goes all the way back to algae?

[Professor Deborah Skeen]

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.

[Jonathan]

That's interesting. Even before we were animals, just sort of when we were like single cell creatures. I mean, 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 our cells.

[Professor Deborah Skeen]

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.

[Jonathan]

Could you talk a bit about how light is setting those clocks?

[Professor Deborah Skeen]

Yeah. Well, this is another surprising thing that I think we all discovered, and that is 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 discovery, showing how light would work to send the signal to entrain. We use the word entrainment or synchronizing our clocks to the outside environment.

[Jonathan]

So there's a link directly from my eye to this master clock telling it, okay, this is the morning or this is the night.

[Professor Deborah Skeen]

Yes.

[Jonathan]

Is it only light that keeps my sort of natural rhythms on track?

[Professor Deborah Skeen]

Well, that's been a big debate. So to get to that, we were studying blind people, totally blind people in some cases where both their eyes have been removed. So we now know the pathway from light to the master clock is disrupted, gone. Then we 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. 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, yeah, is critical and we think the most important based on these studies, in blind people.

[Jonathan]

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?

[Professor Deborah Skeen]

We would all, as a field, 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.

[Jonathan]

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?

[Professor Deborah Skeen]

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.

[Jonathan]

Could you explain to me in simple terms why working at night for a shift might increase my risk of heart disease or cancer? Because I.

[Professor Deborah Skeen]

Right, okay. The first culprit that was produced when we talked about the risks 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 is 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.

[Jonathan]

And these are like the fats, triglycerides are the fats in your blood.

[Professor Deborah Skeen]

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 metabolise 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.

[Jonathan]

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, 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.

[Professor Deborah Skeen]

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 increased risk of these disorders, particularly metabolic disorders, cardiovascular problems and cancer, people have 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 wake 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.

[Jonathan]

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 seeing as much sunlight, it's the fact that my body clock is out of line with, with how I'm living. You know, if I was going to live in a cave for a month, I'm not having the same risk as I am sort of shifting between day and nighttime and my body clock moving around.

[Professor Deborah Skeen]

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. Chrono biologists 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, late types. And people who have fast clocks are more mourning types, or what we call larks. And they all have to fit into this sort of 9 to 5 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 jar and a mismatch that does eventually have some health consequences.

[Jonathan]

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 that some.

[Professor Deborah Skeen]

Yes, this is 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.

[Jonathan]

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 10:00pm yeah.

[Professor Deborah Skeen]

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 the 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 Ronanberg 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. The bigger the mismatch, the more your risk is, the bigger that difference is.

[Jonathan]

So let's say it's two hours. Let's say I wake up at 7am during the week and at the weekends I wake up at nine. I'm at risk of putting on more weight.

[Professor Deborah Skeen]

You are. That's definitely published.

[Jonathan]

Wow, that's extraordinary.

[Professor Deborah Skeen]

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.

[Jonathan]

That's really interesting. I definitely shifted my sleep because suddenly I didn't need to commute.

[Professor Deborah Skeen]

You know, in America especially, they have enormous commute times, getting to work and things. And if you can cut that out and, and use it to sleep, that would be a good health message.

[Jonathan]

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. Is changing?

[Professor Deborah Skeen]

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 photoperiod, 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're all trying to study.

[Jonathan]

Do we have seasonality?

[Professor Deborah Skeen]

Well, we certainly have seasonality in, for example, the incidence of diseases. No better disease to talk about than seasonal affective disorder, or S.A.D. 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.

[Jonathan]

As we're going into the winter, what do we know is like, happening to our body clocks and how might that affect us?

[Professor Deborah Skeen]

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 things that we change in our environment.

[Jonathan]

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 are not cheered 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?

[Professor Deborah Skeen]

And very real? No, this is really where we have strong evidence seasonal affective disorder. So that's in about 1 to 10% of the population. But it depends on the latitude you live at. So of course the higher latitudes, Scotland, Aberdeen, the incidence 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.

[Jonathan]

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.

[Professor Deborah Skeen]

People who have seasonal affective 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.

[Jonathan]

And Deborah, I know you've studied a lot the effects of particular colors of light, and I understand blue light in particular. Why on earth does the color of light matter?

[Professor Deborah Skeen]

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, 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 460-480nm. 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.

[Jonathan]

So the blue light is the thing that's gonna really reset my body clock.

[Professor Deborah Skeen]

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.

[Jonathan]

Got it. So everything will reset my body clock. But the blue light is going to have more impact than the other light.

[Professor Deborah Skeen]

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.

[Jonathan]

I actually think that's a brilliant point to maybe transition to practical advice for listeners. So I think you've sort of painted this picture for why this is all so 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?

[Professor Deborah Skeen]

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 this sort of software that you can put on your or 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.

[Jonathan]

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?

[Professor Deborah Skeen]

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.

[Jonathan]

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.

[Professor Deborah Skeen]

Yes.

[Jonathan]

So if I am going to be looking at these devices And I guess a TV would be the same, therefore, as a phone.

[Professor Deborah Skeen]

Just have your television. So the equation for the impact is, is it's squared. So that 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 that could also reduce the intensity. So it is about getting less light into your eyes at night.

[Jonathan]

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.

[Professor Deborah Skeen]

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 rich 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 that works well for using light if you have to in the morning to keep your clocks synchronized to your eyes.

[Jonathan]

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. 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 outside, still dark outside, or actually it's so gray that doesn't feel very different. What should I do?

[Professor Deborah Skeen]

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.

[Jonathan]

And even in the winter I'm actually getting more light than I, than inside. So you're sort of saying it's actually a lot brighter than I realize and my eyes can see this.

[Professor Deborah Skeen]

It's a lot brighter. It's a logarithmic scale. So it's 50,000 luxury, 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.

[Jonathan]

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?

[Professor Deborah Skeen]

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.

[Jonathan]

I think that'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. There's a lot of nutrition science interest, but you're coming at this from this completely opposite angle about Clarks and I understand you've recently been studying this. So what is the advice you can give us?

[Professor Deborah Skeen]

Everybody has focused for so long on what you should eat, but it's now about when and is there 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 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, whereas light shifts the melatonin rhythm, the food only shifts the glucose rhythm.

[Jonathan]

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. Somehow 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.

[Professor Deborah Skeen]

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.

[Jonathan]

So, Deborah, are you saying that in terms of meal timing and health, consistent timing of meals is important?

[Professor Deborah Skeen]

I think it's gonna 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 there are researchers, you know, at King's that study this and others. So it's this food irregularity that we are all beginning to think isn't good for your body.

[Jonathan]

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 7, 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 7 in order to eat your breakfast.

[Professor Deborah Skeen]

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.

[Jonathan]

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 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 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, probably a hundred years ago now, is it a good idea?

[Professor Deborah Skeen]

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 in the standard time when we go outside at noon and they're more closer together. When we're in pretty 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 like dart cycle and the sun clock and our body clocks and anything that makes that gap bigger with the social clock is gotta be worse for our health. That's where we are now. We just have to prove it.

[Jonathan]

Amazing. I would like to do a quick summary if that's all 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 work, 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 of weight 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, like 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. 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. You said 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 interested in nutrition, 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 getting 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 100 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 effective thing for setting our body clocks or messing up our body clock. And so this idea about putting my phone into a night mode, you know, 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 have really bright light, but it's got less blue, is actually worse than the normal thing, unless 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 been affected. And it sounds like saying everybody's mood has been affected a bit. Some people have this in a. 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 that. 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 7 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 7, 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 our long term health.

[Professor Deborah Skeen]

Yes. Well done. That's a great summary. Please come and be one of my students.

[Jonathan]

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