ZOE Science & Nutrition - Recap: The hidden clock controlling your health | Professor Russell Foster

Episode Date: February 3, 2026

Today, we’re exploring circadian rhythms. Did you know you could eat the exact same meal at two different times of day and your body would respond drastically differently? It sounds like a riddle - but it actually comes down to your body clock.  Every cell in your body has one. A tiny internal ticker, shaped by evolution over thousands of years, to keep you in sync with the phases of the day. So, how can we ensure we’re doing the right things at the right times? I’m joined by Professor Russell Foster to help us all stay in time with our natural rhythm. 🌱 Try our new plant based wholefood supplement - Daily30+ *Naturally high in copper which contributes to normal energy yielding metabolism and the normal function of the immune system 📚Books by our ZOE Scientists The Food For Life Cookbook Every Body Should Know This by Dr Federica Amati Food For Life by Prof. Tim Spector Ferment by Prof. Tim Spector Free resources from ZOE How to eat in 2026 - Discover ZOE’s 8 nutrition principles for long-term health Live Healthier: Top 10 Tips From ZOE Science & Nutrition Gut Guide - For a Healthier Microbiome in Weeks  Better Breakfast Guide Have feedback or a topic you'd like us to cover? Let us know here Listen to the full episode here

Hello and welcome to Zoe Recap, where each week we find the best bits from one of our podcast episodes to help you improve your health. Today, we're exploring circadian rhythms. Did you know that you could eat the exact same meal at two different times of day and your body would respond dramatically differently? It sounds like a riddle, but it actually comes down to your body clock. Every cell in your body has one, a tiny internal ticker shaped by even. over millions of years to keep you in sync with the phases of the day. So how can we ensure we're doing the right thing at the right time? I'm joined by Professor Russell Foster to help us all stay in time with our natural rhythm.

Socadian rhythms, you know, the body clock as it's called, what are they and why do they matter to us? Let's start with why do 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, our 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 popped a plant in a cupboard and would peek in from time to time. Is that right? I never knew that. I assumed it was only the light that caused the flowers to move. That was the assumption. You know, the light, dark cycle drives this rhythmic behavior. But in fact, And he was completely puzzled by this.

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 supra-chiasmatic nuclei, mercifully abbreviated to SCN. And it comprises about 50,000 cells. And when I started in the business, it was assumed that circadian rhythms were the 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. So to make sure I'm following this, this really is the clock, is it? So deep inside my brain is this little thing which is going tick-tock. Is that a way to think about it? I made it too simple? No, no, it is the master clock. Now we know that a bunch of genes, which are turned on, make their proteins. Those proteins form a complex, and then they enter

the nucleus and turn their genes off. The proteins are 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 Uli Shibler, 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 is a, 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 cue. 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? Okay. You ask a really central question. One of the big 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 and cells. But there are also humoral chemical messengers 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're just sort of working our way forward. do know, for example, that the classic sort of 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 the neural connections are very important, but it's not the only thing. There are other chemical messages. What happens if we just say, ah, 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? 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, loss of empathy. We fail to pick up the social signals from other individuals and we lose empathy. Risk taking and impulsivity will do stupid. and unreflective things when we're not synchronized. We 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're 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 air crew are sort of this sort of cardiovascular problems,

heart disease, for example, altered stress responses, lowered immunity. One really interesting set of studies showing that with multiple time zone crosses, the immune system is disrupted, and you're 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 2 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. That doesn't sound very good, Russell. No, it's terrible.

I mean, it's quite clear that this disruption is so much more inconvenient than feeling, you know, tired at the wrong time of day. This has a massive impact. 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? 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're 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 to diabetes. Other experiments have shown that if you load your calories at breakfast and lunchtime versus lunchtime in evening, you know, let's say 2,000 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. It'll often have a 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, 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 of working people, they 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 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 big breakfast, you know, nice lunch, minimum, you know, a bowl of soup in the evening. Thank you for listening to today's recap episode. I want to take 30 seconds to talk about something that's not talked about enough. Menopause.

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