Updated 14th March 2023

How to make your body clock work for you

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    In today’s world, we increasingly live our lives late into the night. Whether it's work obligations, socializing, or being glued to a screen, we ignore the sun's rise and fall. 

    But most of us are unaware that a clock is ticking away inside us. 

    It uses light to keep time and tells us when to sleep and eat. Should we listen? Or is this biological clock simply a relic from a past time?

    In today’s episode, Jonathan speaks to a leading authority to unravel the latest science revealing that this body clock, or circadian rhythm, has a dramatic impact on our health, weight, and mental well-being.

    Russell Foster is an Oxford University scientist who has dedicated his life to studying circadian rhythms. He’s a world leader in the field and the author of Lifetime: The New Science of the Body Clock. 

    If you want to uncover the right foods for your body, head to joinZOE.com/podcast and get 10% off your personalized nutrition program.


    [00:00:00] Jonathan Wolf: Welcome to ZOE Science and Nutrition, where world-leading scientists explain how their research can improve your health.

    In today's world, we increasingly live our lives late into the night. Whether it's work obligations, socializing, or being glued to a screen, we ignore the sun's rise and fall. But most of us are unaware that a clock is ticking away inside us. It uses light to keep time and tells us when to sleep and eat. So should we listen, or is this biological clock simply a relic from a past time?

    The latest science reveals that this body clock or circadian rhythm has a dramatic impact on our health, our weight, and our mental wellbeing. Even our immune system has a strong 24-hour cycle. Today, you'll find out how your circadian rhythm impacts your life and learn some simple steps you can take to be more alert, sleep better and lower your risk of disease.

    I'm joined by Professor Russell Foster. An Oxford University scientist who has dedicated his life to studying circadian rhythms. He's a world leader in the field and the author of “Lifetime, the new science of the body clock”. Russell, thank you so much for joining me today. Why don't we as always start with a quick fire round of questions from our listeners.

    And the first question was, are circadian rhythms important for our health? 

    [00:01:35] Prof. Russell Foster: Incredibly important. 

    [00:01:37] Jonathan Wolf: All right. If I take medication at a different time of day, will it change how effective the medication is? 

    [00:01:45] Prof. Russell Foster: Yes. There are over a hundred different drugs with a known time of day effect. 

    [00:01:49] Jonathan Wolf: Which I think is extraordinary. Is exposure to light from digital devices in the evening, really bad for us?

    [00:01:57] Prof. Russell Foster: No. 

    [00:01:59] Jonathan Wolf: Oh, that's gonna be a shock. We'll definitely dig into that later. Is it possible for me to improve my sleep without drugs? 

    [00:02:06] Prof. Russell Foster: Absolutely. 

    [00:02:07] Jonathan Wolf: Should we let our teenagers sleep in? 

    [00:02:10] Prof. Russell Foster: Ideally not, but you've gotta be pragmatic. 

    [00:02:14] Jonathan Wolf: Okay. And finally, is it true that some people are night owls who can stay out later than the rest of us?

    [00:02:20] Prof. Russell Foster: Absolutely true. 

    [00:02:21] Jonathan Wolf: Wonderful. Well, look, I'm really looking forward to getting into that. I think there are a whole bunch of answers that many of our listeners already are gonna be quite surprised about. Russell. What is the biggest myth about our body clock, that drives you crazy?

    [00:02:34] Prof. Russell Foster: Oh, Jonathan, I think that's fairly straightforward, which is that we are all the same and that we all have the same temporal structure.

    And that does drive me crazy because of course we have the concept of the chronotype, which is essentially you can be a morning person, an intermediate person, or an evening person. This changes as we age, this changes depending upon our genetics, this changes, depending upon whether we see morning, light or evening light, we are very much not the same creature and there's a huge diversity across humans, you know, and we probably all know from our own friend circle or family circle, there are people that like to get up early and go to bed early. And at the other end of the spectrum, people love to go to bed late and get up late. 

    [00:03:17] Jonathan Wolf: Brilliant, really wonderful to have you here.

    Circadian rhythms, you know, the body clock as it's called, what are they and why do they matter to us?

    [00:03:25] Prof. Russell Foster: Let's start with, why they matter. If you think about our biology what it has to achieve is the right substances at the right concentration delivered to the right tissues and organs at the right time of day.

    And it's the sort of temporal time structure that is delivered by our circadian system, the body clocks that allow us to do this, that allow us to essentially optimize our biology. So they're incredibly important. Essentially. They influence every aspect of our physiology and behavior. What they are has turned out to be really fascinating. We've sort of known about rhythmicity for a very long period of time. The first sort of description of a circadian rhythm goes back to about 1729 in plants with a rhythmic opening and closing of leaves under constant conditions under constant darkness. In fact, this French astronomer hopped a plant in a cupboard and would peek in from time to time.

    [00:04:23] Jonathan Wolf: Is that right? I never knew that I assumed it was only the light that caused the flowers to move. 

    [00:04:27] Prof. Russell Foster: That was the assumption, you know, the light-dark cycle drives this rhythmic behavior, but in fact, and he was completely puzzled by this. Like, there must be some internal mechanism and he was dead right. And it took us quite some time to try and understand what that mechanism was.

    I mean, in the early days we knew that there was a master clock within the brain. In the hypothalamus, an area called the suprachiasmatic nuclei is mercifully abbreviated to SCN and comprises about 50,000 cells. And when I started in the business, it was assumed that circadian rhythms were a product of cell-cell interactions, a sort of a circuit property that would generate a 24-hour oscillation in electrical activity.

    And then, I mean, it was so exciting because people had isolated individual SCN neurons and shown that they would tick away in a dish on their own in terms of electrical activity and, indeed turning on and off of genes. 

    [00:05:23] Jonathan Wolf: So to make sure I'm following this. This really is the clock. Is it so deep inside my brain? Is this little thing that is going tick-tock. Is that a way to think about it or have I made it too simple?

    [00:05:33] Prof. Russell Foster: No, no, it is the master clock. And now we know that a bunch of genes that are turned on, make their proteins, those proteins form a complex, and then they enter the nucleus and turn their genes off. The proteins have then degraded and then the whole cycle starts again. So there's a molecular feedback loop and we thought that it was only SCN cells that had this capacity and then a wonderful chap called Ueli Schibler, who's based in Switzerland showed that actually lots of different sorts of cells have a clock. So you can think of a master clock within the brain coordinating the rhythmic behavior of literally billions of individual cellular clocks throughout the organ systems of the body. So you've got this incredible hierarchy.

    I mean, it's sort of a bit, the analogy I've sort of used, it's a bit like the master clock in the brain is a conductor of an orchestra. She is producing a rhythmic temporal signal from which the rest of the orchestra, the body, takes its reference. 

    [00:06:34] Jonathan Wolf: And so we're not flowers. So I guess it makes sense that the sun goes away.

    They close and the sun comes up, but we're not flowers. Why is this so important to us? Like, why do we need to have this clock and this change? Why aren't we just sort of constantly on, like my computer, right? It doesn't change. 

    [00:06:51] Prof. Russell Foster: If you think about it, we sit on a planet that revolves once every 24 hours.

    And that revolution produces profound changes in light and temperature, and essentially all life on the planet, almost all life, including some types of bacteria have evolved a clock that can anticipate these different changes and gear up physiology and behavior in advance of the change. Let's say, in advance of when we wake up in the morning, our glucose metabolism is increasing.

    Our stress hormones are rising. Our blood pressure is rising in anticipation of increased activity when we actually are awake. Now, if we just waited until the lights came on. It was dawn. If we then triggered this process, we would've wasted two to three hours of transition where we wouldn't be able to work effectively and optimally.

    So what a clock is allowing us to do is anticipate these predictable changes in the environment and get our physiology up and prepared in advance. So we don't waste any of the opportunities that the changing environment provides. 

    [00:07:55] Jonathan Wolf: Got it. So what you're saying is we shouldn't think of ourselves as like a computer or a car, which you turn on as instantly available to do whatever the thing is.

    Actually, you know, it can take us hours to be in the optimal place to deal with the stress of trying to convince my girl to go to school in the morning. I can't just switch it on is what you're saying. 

    [00:08:12] Prof. Russell Foster: Yep.

    [00:08:12] Jonathan Wolf: I need a couple of hours of prep to be ready to deal with that. And that is true for many of the processes that go in our bodies.

    And so you need to know sort of where you're going to be in a few hours. It's no good to just be like, oh, off, on, you know, just deal with the environment as it is. 

    [00:08:28] Prof. Russell Foster: Well, that's right. I mean, again, you could think of it a bit like an engine. It will be like going from first gear into reverse, you know, you can't do it.

    You have to go through the stages. Otherwise, you destroy the engine and, you know, it's so important to appreciate that the different states of wake and sleep are just so very, very different. Our metabolism is utterly different. Our ability to process food, our drugs. Everything is changing our cognitive abilities.

    Our ability to process information in the early hours of the morning is worse than if we consume sufficient alcohol to make us legally drunk. We're that impaired! 

    [00:09:06] Jonathan Wolf: So say that again, Russell. You're saying first thing in the morning, I am as impaired as if I was legally drunk? 

    [00:09:11] Prof. Russell Foster: Around about four to five o'clock in the morning.

    Yes. Before the clock has started to wake you up, your ability to process information is impaired. And so, you know, the serious point is if you're driving a car at four, five o'clock in the morning, as many people have to do after coming home for the night shift or getting the family off to that sort of ferry and the early holiday, your ability to drive that car is worse than if you consume sufficient alcohol to make you legally drunk.

    So it's a big deal. 

    [00:09:39] Jonathan Wolf: That's amazing. And I think many of us assume that this is just because we're really tired, right? So, you know, four in the morning, most of the time we probably haven't gone to sleep or we've woken up incredibly early. I think you're saying it isn't just because I'm tired. 

    [00:09:54] Prof. Russell Foster: Yeah. 

    [00:09:55] Jonathan Wolf: It's actually to do with the body clock's point I'm in, is that correct?

    [00:10:00] Prof. Russell Foster: Exactly. And in fact, if you haven't slept, it gets worse. So as a double whammy, and often we refer to sleep and circadian rhythm disruption, scarred because it's very difficult to, in many cases tease apart the two. And in fact, what sleep deprivation will do is actually make the sort of impairment by the clock even worse.

    So that 4:00 AM window, where you are naturally at your lowest cognitive abilities will be made worse if you hadn't slept properly over the preceding few days. 

    [00:10:31] Jonathan Wolf: There'll be a lot of people listening to this call who do shift work. So they're not on a regular pattern and there are others who travel across time zones.

    And we had a lot of questions for both of these. So I think all these people know how powerful the body clock is. And I think anyone listening who's ever gone across many time zones has sort of had that strange experience. How does your body synchronize between these? So how does my body know that it's now the middle of the afternoon and not in the morning?

    [00:10:58] Prof. Russell Foster: Okay. So we talked about this master clock regulating this sort of billions of individual cellular clocks and this temporal structure is of absolutely no adaptive value, fine-tuning biology to the demands of the day and the night, unless it's set to the external world. And so we know that the light-dark cycle, the dawn-dusk cycle is the most important signal and there are receptors in the eye which detect this light and then regulate the internal molecular clockwork. Now what's turned out to be extraordinary, is that the receptors in the eye that detect light for the clock are different from the ones that we use for vision. So the visual cells, the rods, and cones give us our sense of space.

    But we discovered that this third photo receptor within the eye is a photosensitive retinal ganglion cell. So the cells within the retina that form the optic nerve are called the ganglion cells. And one out of every hundred of those cells is directly light sensitive and projects to the master clock within the brain.

    [00:12:03] Jonathan Wolf: And I think this has been some of your own work, hasn't it? Sort of untangling exactly this. How long have we known that these extra senses exist? Is this something we've known about for hundreds of years? 

    [00:12:13] Prof. Russell Foster: No, no, not at all. In fact, when we first sort of proposed it in about '91 and it took about a decade to convince my colleagues. I remember in one presentation I gave, I sort of said, so these data are consistent with the fact that there's another receptor within the eye. And somebody at the back stood up and looked at me and I thought they were gonna ask a question and they just shouted bulls**t and it was quite shocking as a young researcher, but in a sense, what it does is bludgeon you into doing better and better experiments.

    [00:12:44] Jonathan Wolf: Just as an aside for a minute, I think many people listening to this call will be really surprised because they won't have been involved in the process of scientific discovery. And I think there is this idea that it's all carried out in an extremely civilized fashion. Everybody just looks at the data and says, this is, you know, the, you're laughing already, but actually I have rarely seen, you know, two people more impassioned than two experts in the same area, disagreeing about something that almost nobody else in the world actually even understands.

    [00:13:14] Prof. Russell Foster: Well, it is like that. And of course, what it does is it drives you to do better experiments and then produce an iron cast set of data, supporting your hypothesis. And so, yeah, we finally, in a couple of back-to-back science papers in 1999, convinced the world, I think that there was another receptor and that led to a lot of interest from other groups around the world.

    We showed it in the mouse. Other people showed it in the rat and, and monkeys that there is this photosensitive retinal ganglion cell. And so, you know, it's so exciting because you kind of thought that the eye was the best understood bit of the central nervous system. We studied it for 150 years. Surely we can't have missed an entire class of light sensors.

    Well, we had, and I think that's part of the joy of doing science.

    [00:14:00] Jonathan Wolf: So you've explained like we've got this master body clock, it sounds like light is central for helping to keep it on track. Can you help to explain a bit how this body clock then influences our body? And I guess the most obvious thing to start with is sleep, which I think is what we think about. So how does this fit together? 

    [00:14:18] Prof. Russell Foster: You ask a really central question. One of the bigger questions in the field at the moment is, you know, what are the signals from the master clock that are coordinating the rest of the body? And we know that the sympathetic nervous system, that bit of the nervous system that is sort of under involuntary control, that is regulating sort of heart rate and a whole bunch of other sorts of things.

    It's via those projections, which are influencing the activity of organs, the cells, but there are also humoral, chemical messages, that are produced by the master clock that are regulating various tissues. So we don't fully understand there are direct, nervous connections. There are hormonal outputs, and we are just sort of working our way forward.

    We do know, for example, that the classic sort of the rhythmic change in metabolism that you see is if you cut the nervous connections to key organs, such as the liver, for example, then the rhythmic changes in the liver don't occur. So we know that neural connections are very important, but it's not the only thing. There are other chemical messengers.

    [00:15:18] Jonathan Wolf: And so what happens if we don't live alongside our circadian rhythm? So you've described this pattern where it's getting me all ready and it makes me ready for breakfast, and it makes me ready to go out and do things and, and ready for sleep. What happens if we just say "augh", you know, the circadian rhythm was for our ancestors, I've got electric light now, and I've got fun things to do.

    What happens if I ignore it? 

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    [00:15:40] Prof. Russell Foster: And of course, we go back to your original question, which I didn't answer, which is what happens with jet lag and shift workers. And of course, you eventually get over jet lag because you are exposed to light in your new time zone picked up by those amazing receptors that lock then the internal clock onto the new light-dark cycle.

    But it can take a day for every time zone you've crossed to fully synchronize. So it can take quite some time. And the question many people ask of course is, well, why don't night shift workers adapt to the demands of working at night that have always been the assumption, but the problem again is light. So in the factory or the workplace, there are relatively dim light levels, but then the individual will experience bright, natural light on the drive home, or indeed on the drive into work.

    And the clock will always defer to the brighter light signal, which is invariably the environment as daytime. And so in fact, the clock of night shift workers and day shift workers is broadly the same. We're locked onto the same light-dark cycle. Now experiments have been done where individuals have been exposed to brighter light in a workplace.

    About 2000 lux, which is a measure of light intensity. And they'd actually hidden from light during the day. And just like jet lag, they eventually adapted to the night shift. The problem is that's not a practical solution for most individuals. So that's the mechanism, but what are the problems? And what's emerged, I think over the past few years is the appreciation that our sort of emotional responses, our cognitive responses, and our physiology and health can be profoundly affected by not doing the right thing at the right time. So in terms of emotional responses, we see fluctuations in mood irritability and loss of empathy.

    We fail to pick up the social signals from other individuals and we lose empathy, risk-taking, and impulsivity. We'll do stupid and unreflective things when we're not synchronized. Also, combined with sleep loss, the brain, forgets the positive experiences and remembers the negative ones. So circadian rhythm disruption, and sleep loss means that you are basing your decisions upon your negative rather than your positive memories, which I think is so important.

    And then, of course, you know, you can slide into using excessive amounts of caffeine to keep you awake, and then trying to reverse that when you want to go to sleep with excessive alcohol. There's the failure to multitask, your concentration goes, your communication, your decision-making skills all fall apart, and you again, lose social connectivity.

    So those are the short-term things that many of us have experienced to some degree, but what you get in long-term night shift workers and indeed individuals such as long haul pilots and aircrew are sort of this sort of cardiovascular problems, heart disease, for example, altered stress responses, lowered immunity.

    One really interesting set of studies shows that with multiple timezone crosses, the immune system is disrupted and you are less able to fight off bacterial infection for example, and it may be the basis for why there are higher rates of cancer in long-term night shift workers, very important studies on night shift nurses showing higher rates of breast cancer, colorectal cancer, for example, and in long haul pilots, prostate cancer, metabolic abnormalities, such as diabetes, two and obesity. And finally, the big one, depression and psychosis are made much worse by sleep and circadian rhythm disruption. So to answer your question briefly...

    [00:19:24] Jonathan Wolf: That doesn't sound very good, Russell.

    [00:19:26] Prof. Russell Foster: No, it's not as terrible. I mean, it's quite clear that this disruption is so much more inconvenient than a feeling, you know, tired at the wrong time of day. This has a massive impact. 

    [00:19:37] Jonathan Wolf: Well, it's interesting, isn't it? Because I think that's what you tend to think about is, you know, I've had a bad night or it's really tough to be doing the night shift.

    I just wanna clarify this because are you saying that even if I was sleeping properly, so let's say I'm doing a night shift, I've done it for long enough or I'm good enough at it that I am getting sleep during the day. So it's not that I'm not sleeping. These things are actually a result of being against my circadian rhythm. They're not just because I'm tired and lack of sleep is causing this. 

    [00:20:07] Prof. Russell Foster: Well, it's the double whammy. So essentially you are on the night shift. Your whole biology is saying you should be in the sleep state. So what do you do about it? Well, the body's response is to over-activate the stress access so that you can function at least at some level, you can't expect peak performance incidentally, but you can function at a certain level for a certain amount of time.

    The problem is the night shift worker then who's chronically sleep deprived and who's worked against their body clock. Then they go home and they try and sleep. And the clock is saying, hang on, no, it's daytime. You should be awake. So the sleep pressure is very high and it will induce a certain level of sleep, but it's not coordinated with the clock.

    So the sort of quality and the duration of the sleep that you get during the day is not comparable to the sort of sleep that you get under a natural sort of sleep break. 

    [00:20:55] Jonathan Wolf: So you can't necessarily completely separate the two because they fit together so much is what you're saying, sleep and circadian rhythm, this is one of the fundamental things that should be happening on this 24-hour, hour clock. 

    [00:21:07] Prof. Russell Foster: Yeah. And these two systems interact, which is why we came up with the term scarred sleep and circadian rhythm disruption. Because classically, as in the case of night shift work, it's very difficult to disentangle the different contributions of sleep deprivation and circadian misalignment.

    And in fact, they reinforce each other and make it much worse. 

    [00:21:26] Jonathan Wolf: That is a rather miserable story for listening to this. And you are getting ready to go to your shift work, which lots of people listening to this podcast will do. I just wanted to dig in a minute to some of those things, because they sounded quite scary.

    Many of the people who do shift work after all are working in healthcare. So there will be nurses and doctors and they will be having to make life and death decisions in the middle of the night. Right? So I luckily never have to do this, but that happens. You said something about risk-taking and the brain only remembering the sort of negative memories.

    Could you explain a bit more sort of what's going on there and then I guess, is there anything we can do about it if you are in these situations? 

    [00:22:05] Prof. Russell Foster: So, for example, after extended periods of time on the night shift, your vigilance drops hugely. And, and this can impact the safety of your journey home. A study from the UK showed that 57% of junior doctors had either had a crash or a near miss on the drive home after the night shift.

    So what can we do? Well, we can provide vigilance devices that clip onto the dashboard that will map head nods or eye-roll. And in fact, high-end German cars are building this technology routinely so that if you are likely to fall asleep, there's an alarm that goes off and wakes you up now, that's a sort of a simple life-saving device.

    You can increase the amount of light in the workplace because increased light increases alertness and makes you less likely to fall asleep. And so we need to look very carefully at light levels in the workplace itself. Knowing that there's poor physical and mental health associated with things like night shift work, we could institute, high-frequency health checks for the workforce, catching potential, you know, early risk cancers, obesity, diabetes 2, before these become chronic and potentially irreversible conditions. This is simple stuff, knowing that there are high rates of obesity, diabetes 2, and metabolic abnormalities. What kind of food do we provide to our night shift workers? Well, it's fast food, high-fat, high-sugar. We should actually make available for our night shift workers, protein-rich, easy-to-digest snacks. That's the sort of thing that the body can cope with and is not gonna predispose to these metabolic abnormalities.

    I think also many night shift workers don't appreciate some of the consequences of what's going on and it's not just those individuals, but it's also the people they share their lives with. So studies from the States have shown that in some sectors, the divorce rate is six times higher in night shift workers compared to day shift workers. So we've got to provide education so that people understand the consequences. They can look out for those problems, not just in themselves, but so that their partner will appreciate that the person, this wonderful, loving person, they married, hasn't turned into a monster. It's an inevitable consequence of doing this particular job. So education is important. 

    [00:24:25] Jonathan Wolf: And Russell, that sounds all quite profound. So it sounds to me, you know, maybe there are other fellows on this, we should be trying to minimize the number of people who are having to work in conflict with their body clock. These are not minor things you're talking about. Right? These are pretty shocking. 

    [00:24:39] Prof. Russell Foster: I think that's right. And I think that just because we can, should we? And I think we need to ask that question very carefully. And of course, so many people don't want to do the night shift. It's an economic driver, for many women, for example, many nurses, the only way to have a family and a career often is to do night shift work. So you can get the kids off to bed. You go on the night shift, you're back to get them off to school, and then you struggle, you know, trying to sleep during the day and on all the rest of it. So there are really severe economic drivers, but we need to try and mitigate some of this risk where possible we need to avoid it.

    But there are unavoidable areas. As in our frontline staff, our policemen, our ambulance crews, our doctors and nurses, a very interesting study, is even adding yet another whammy to our frontline staff. And particularly during the recent COVID, it's been shown that vaccination, is far less effective, you don't get, a robust antibody response if your circadian rhythm is disrupted immediately before the vaccination or afterward, probably because of increased levels of stress hormone.

    Those stress hormones will suppress the immune system and therefore the vaccination isn't as effective. So before we vaccinate our frontline staff, we should ensure that they've had a period of rest and relaxation immediately prior to the vaccination. 

    [00:26:02] Jonathan Wolf: And presumably that suggests that if I've messed up my body clock at this point, then just my immune system, in general, is not going to work as well, which is bad, not only for infectious diseases, but we know for, you know, things like cancer and all the rest of it.

    [00:26:15] Prof. Russell Foster: Yeah, absolutely. Some very interesting studies. And of course, this needs to be built into medical programs. I was talking about this at the weekend, at a meeting that a stable circadian sleep-wake timing will increase outcomes and long-term survival in cancer patients. And the difference between stable sleep-wake versus disrupted sleep-wake was something like in two years, 70% survival versus 40% survival.

    So these are big effects that we should be building into our health programs. 

    [00:26:44] Jonathan Wolf: That's amazing. So, Russell, I'd love to talk about how the circadian rhythm is changing during our lives. So is it true that there's a real change in your circadian rhythm? At the point when, for example, you go through menopause, which is where we've had a lot of questions.

    [00:27:00] Prof. Russell Foster: Yeah, the changes during menopause are related primarily to the changes in the estrogen and, and progesterone those two key hormones that, cycle across the menstrual cycle. Now estrogen has been associated with increased sleep, broadly speaking and progesterone, greater relaxation. So these are really important hormones.

    And so in fact, you know, you get many women to find just around the time of ovulation, they're more relaxed. They get better sleep, but premenstrual before menstrual bleeding, where estrogen and progesterone alone, then you get greater levels of anxiety and potentially depression and irritability. And so there are those premenstrual things.

    [00:27:39] Jonathan Wolf: And is that actually affecting the body clock at this time? Or is this... 

    [00:27:43] Prof. Russell Foster: So it's probably a more direct effect of the hormones. What we do know is that the circadian system is fundamentally important in the release of the neuro-hormones in the brain, regulating the pituitary gland, the release of the key hormones from the pituitary gland that then arrive at the ovary, and indeed test this, and indeed the response of those organs to those hormones is all being regulated by the clock, which is why in again, night shift workers and indeed aircrew you find disrupted menstrual cycles, more difficulty getting pregnant. And indeed there's a statistical, although not hugely, there not been many studies, but a greater risk of miscarriage.

    And in fact, if you get undergoing IVF, the recommendation is not to do lots of transmeridian flights or indeed do night shift work. So we know the clock is extremely important in reproduction. In males, for example, there's a morning rise in testosterone and it's at that time of day, that sperm motility and sperm activity is the most efficient. Whether that directly correlates with conception is unclear, but certainly, you know, you predict that it would be important. So the clock is embedded in a lot of these things, but in terms of menopause, it's more of a direct effect of those changing hormones, which are driving a temperature, relaxation, and sleepiness, which is a great problem. And what's so frustrating is that this is such an important impact on women, and yet there's very little intervention. There's very little that one can do about it. There have been studies that have suggested that if you use good sleep hygiene, the sorts of things I've just talked about, then you can actually get better sleep and you can sort of get through menopause and the premenopause perhaps more effectively, but there's no magic bullet.

    Sadly, HRT, in some women has been shown to improve sleep, but not for all. 

    [00:29:41] Jonathan Wolf: Is there a circadian rhythm for our guts? Clearly, we talk a lot about nutrition and the microbiome. Is there something that the circadian rhythm has to tell us about how we should think about eating? 

    [00:29:52] Prof. Russell Foster: Yes, and this is a really important area.

    So the empirical evidence shows that if we have a constant infusion of glucose throughout the day, our ability to clear that glucose is much more efficient during the first half of the day. And it tails off towards evening and then becomes pretty inefficient. So in fact, you know, getting rid of glucose very effectively during the first half of the day, But less effectively during the second half of the day.

    And of course, by having higher levels of glucose, you can generate glucose intolerance and you're on the way to type 2 diabetes. Other experiments have shown that if you load your calories at breakfast and lunchtime versus lunchtime and evening, you know, let's say 2000 calories, which were designed to promote weight loss, the weight loss was much more effective.

    During the first half, if you had your calories during the first half of the day, rather than the second half of the day. So there's this circadian modulation of our metabolism. So you think, well, hang on, this is crazy. I mean, society now often misses breakfast. We'll often have lunch at the workplace.

    And then finally, after the commute home, you have your large meal, but this large meal in the evening is a recent occurrence in human behavior. In the medieval period, the breakfast and lunch, you know, those great banquets we all think of, you know, in the Tudor period were at lunchtime, they weren't in the evening.

    And so our changed eating habits have been relatively recent driven by the aristocrats as so a show of wealth was illuminating your house with candles and it's worth bearing a mind until, you know, 19th century a candle was the equivalent of a working man's daily wage. They were incredibly expensive. And so many working people followed the natural light-dark cycle.

    Whereas the aristocrats started eating later and that sort of trickled down in more recent years to much of society eating later in, in the evening. And that's the worst possible thing, that we could be doing. So if possible, and I, again know it's complicated because of the way we structure our lives and the commute and all the rest of it.

    Try a big breakfast, you know, nice lunch, a minimum bowl of soup in the evening. 

    [00:32:11] Jonathan Wolf: And I think I would just add that we see this is quite personalized. We definitely see that these average in many of these experiments done. And there are people actually, we, where we see quite flat responses and others where we see this most strongly.

    Interestingly, on average, you see this change with age and we haven't separated that from circadian rhythm, but I guess part of what you're suggesting is if your circadian rhythm is maybe not as strong as it was, then also your control between this may not vary as much across this time. 

    [00:32:38] Prof. Russell Foster: But Jonathan also raises a very important point because most experiments like the sort I've discussed on the metabolism have been undertaken in university undergraduates, and volunteers and they've usually been males.

    And so it's good that you raise this because many of the times we are extrapolating from healthy young males to the rest of society, including females. And that simply may not be appropriate under certain circumstances. So yeah, the data are great for healthy young males. But we need studies on other demographics and also between men and women.

    [00:33:12] Jonathan Wolf: Final question, I'd like to ask, because we've got a lot of questions about this. What about those of us who have, or have had small children waking us up in the night? Sometimes for very long periods of time, that seems to be disrupting one circadian rhythm quite badly. Is that going to damage our long-term health in the same way you described as night shift?

    And if so, what do we do about it?

    [00:33:32] Prof. Russell Foster: It's unclear, you know, for such a prevalent experience, there's not been a huge amount of research on it. So I suppose what I would say is that the first few weeks and months, when a newborn, you know, arrive because of the need to pack on the calories, then there will be the need to feed throughout the night.

    But that diminishes fairly quickly. The trouble is that many young children become very dependent upon that type of physical contact. And they will wake up and seek comfort, whereas actually, they don't need food. And so what I think has to be, and it's very much a personal issue, you have to then sort of start being fairly disciplined and developing a robust sleep structure for the youngster and the sorts of things that we've been talking about, you know, dark bedrooms and the right temperature and a very clear structure. And again, I'm cautious in this regard because I'm not a pediatrician, but I think you have to be very careful about always going to the child, making sure the child is okay, but they will seek attention and it's not necessarily always good for them.

    And it's not necessarily always good for the parent. 

    [00:34:46] Jonathan Wolf: And this is because your message is fundamental, getting to a really healthy body clock is so important for our health. 

    [00:34:53] Prof. Russell Foster: Yeah.

    [00:34:53] Jonathan Wolf: And so, as we think about things we want to do for our children, as well as for ourselves, you're basically saying on average that this is one of the things that you want to try and gift your children.

    [00:35:03] Prof. Russell Foster: That's right. I, I mean, I know others would disagree, but that would be my personal view. 

    [00:35:08] Jonathan Wolf: Russell, thank you so much. I think there are so many topics that I wanted to get into, but we haven't been able to cover all of them this time we always try and do on this podcast is for me to try and do a quick summing up of the many different things we've covered.

    And we started by saying just how important the body clock is. And this matters because if we're living outside of the way, the body clock setting, there are a lot of negative outcomes. And that if we move outside this. And some of us have to because for example, we're night shift workers, they're a very profound impact.

    And I think you also said that as we go through aging, this tends to mean that the circadian rhythm is not as good. And in particular, we talked about women going through menopause where you'd have not only the circadian rhythm but really these changes in these hormones leading to really a significant disruption of sleep, which obviously has a lot of the impact that you were talking about.

    And I think the other key takeaway for me is just how personalized a lot of things are that you've talked about. You know, another example, whether it isn't just, just a one size fits all approach. 

    [00:36:06] Prof. Russell Foster: Absolutely. And different doesn't have to be bad stop worrying about one sick sleep and enjoy it and embrace it.

    [00:36:13] Jonathan Wolf: Brilliant. Russell, thank you so much. I enjoyed it very much. I really enjoyed the book as well, and I hope we'll get you to come back again and talk to us about some more of the ongoing research. 

    [00:36:24] Prof. Russell Foster: I'd love to, and of course, we haven't even touched on when to take your medications for the best effect. See if we can unpack next time.

    [00:36:30] Jonathan Wolf: Let's do that. Maybe a whole podcast just on that. 

    [00:36:32] Prof. Russell Foster: Okay. 

    [00:36:32] Jonathan Wolf: Russell. Thank you so much.

    [00:36:34] Prof. Russell Foster: Great pleasure. Thank you. 

    [00:36:35] Jonathan Wolf: Byebye. 

    Thank you to professor Russell Foster for joining me on ZOE science and nutrition today. We hope you enjoyed today's episode. If you did, please be sure to subscribe and leave us a review as we love reading your feedback.

    If this episode left you with questions, please send them in on Instagram or Facebook. And we'll try to answer them in the future episode. At ZOE, we want to improve the health of millions by understanding the right food for each of us to improve our health and manage our weight. Each member starts with an at-home test, comparing them with participants in the world's largest nutrition science study.

    If you're interested in learning more about ZOE, you can head to joinZOE.com/podcast and get 10% off your personalized nutrition program. As always, I'm your host, Jonathan Wolf, ZOE Science and Nutrition is produced by Fascinate Productions with support from Sharon Feder, Yella Hewings-Martin, and Alex Jones here at ZOE.

    See you next time.

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