Recap: The hidden clock controlling your health | Professor Russell Foster

Summary

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Episode Overview

Podcast: ZOE Science & Nutrition
Episode: Recap: The hidden clock controlling your health
Host: Jonathan Wolf
Guest: Professor Russell Foster
Release Date: February 3, 2026

This episode explores the science of circadian rhythms—your body’s internal clock—and how profoundly it influences everything from your sleep and mood to metabolism and long-term health. Professor Russell Foster, a leading expert on circadian biology, joins to discuss where the "body clock" resides, how it orchestrates physiology, the risks of ignoring it, and the surprising connection between meal timing and metabolic health.

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Key Discussion Points & Insights

What Are Circadian Rhythms and Why Do They Matter?

• Cellular Internal Clocks: Every cell has a built-in "clock" evolved over millions of years to help bodies function optimally at different times of day.
• Coordinated Timing: The circadian system makes sure “the right substances at the right concentration [are] delivered to the right tissues and organs at the right time of day.” (Russell Foster, 00:52)
• Historical Discovery: Awareness of circadian rhythms dates back to 1729, when a French astronomer observed that plants opened and closed leaves on a regular rhythm, even in darkness, hinting at an internal timer. (01:50)
  • Notable Quote:
    “This French astronomer popped a plant in a cupboard and would peek in from time to time.” (Russell Foster, 01:50)

The Master Clock in the Brain

• Suprachiasmatic Nucleus (SCN): The SCN in the hypothalamus contains ~50,000 cells and is known as the "master clock" that synchronizes all body clocks.
  • “You can think of [SCN] as a conductor of an orchestra—she is producing a rhythmic signal from which the rest of the orchestra, the body, takes its reference cue.” (Russell Foster, 03:35)
• Molecular Mechanism: Clock genes are turned on, make proteins, these proteins then shut off their own gene—a molecular feedback loop that creates a roughly 24-hour rhythm. (03:00)

How the Body Clock Influences Physiology

• Connection Pathways:
  • The SCN sends signals via the sympathetic nervous system (controls heart rate, etc.) and chemical messengers (hormones) to coordinate rhythms in all organs.
  • Direct neural connections are crucial: disrupting them (e.g., to the liver) wipes out rhythmic changes in those organs. (04:12 – 05:13)

What Happens If We Ignore Our Circadian Clock?

• Short-term Effects:
  • Mood swings, irritability, loss of empathy, poor decision making, decreased multitasking and social skills, increased risk-taking. (05:21)
  • “Risk taking and impulsivity—we do stupid and unreflective things when we're not synchronized.” (Russell Foster, 05:51)
  • Tendency to rely on caffeine and alcohol for alertness or sleep—further destabilizing rhythms.
• Long-term Effects:
  • Significant impacts documented among shift workers, pilots, and night nurses:
    • Higher rates of cardiovascular disease, cancer (breast, colorectal, prostate), diabetes, obesity, lowered immune response, depression, and psychosis.
    • “What you get in long term night shift workers… is cardiovascular problems, altered stress responses, lowered immunity...” (Russell Foster, 06:54)
  • Notable Quote:
    “This disruption is so much more inconvenient than just feeling tired at the wrong time of day. This has a massive impact.” (Russell Foster, 07:45)

Circadian Rhythms and the Gut: Nutrition Timing Matters

• Glucose Metabolism:
  • The body clears glucose most efficiently in the morning and early afternoon; the same meal eaten at night leads to higher blood sugar.
  • “Our ability to clear that glucose is much more efficient during the first half of the day… it tails off towards evening.” (Russell Foster, 08:02)
• Weight Loss Studies:
  • Shifting caloric intake to the morning and lunch (rather than lunch and dinner) promotes greater weight loss, likely due to the circadian modulation of metabolism.
  • “If you load your calories at breakfast and lunchtime… the weight loss was much more effective.” (Russell Foster, 08:47)
• Modern Eating Patterns:
  • The late-evening meal is a recent societal development, introduced by the elites and now common for everyone due to work and commuting patterns.
  • “This large meal in the evening is a recent occurrence in human behavior… in the medieval period… great banquets were at lunchtime.” (Russell Foster, 09:26)
• Practical Tip:
  • Try to eat a big breakfast, a substantial lunch, and just a small, light meal in the evening if possible, to align with your body’s clock. (10:19)

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Notable Quotes & Memorable Moments

• On why the circadian clock matters:
  “Essentially, they influence every aspect of our physiology and behavior.” (Russell Foster, 00:58)
• On the orchestra analogy:
  “The master clock in the brain is a conductor of an orchestra… from which the rest of the body takes its cue.” (Russell Foster, 03:35)
• On the risks of ignoring circadian rhythms:
  “Depression and psychosis are made much worse by sleep and circadian rhythm disruption.” (Russell Foster, 07:33)
• On societal eating habits:
  “It’s the worst possible thing that we could be doing… Try big breakfast, nice lunch, minimum bowl of soup in the evening.” (Russell Foster, 10:19)

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Timestamps for Important Segments

• 00:01 – Introduction to circadian rhythms and body clocks
• 00:52 – Importance of circadian timing in biology
• 01:50 – Historical discovery of internal body clocks
• 03:00 – 04:01 – Explaining the master clock and its orchestration of cellular clocks
• 04:12 – 05:13 – How the body clock influences sleep and overall physiology
• 05:13 – 07:45 – Consequences of ignoring or disrupting circadian rhythms
• 08:02 – 10:19 – How circadian rhythms affect metabolism, blood sugar, and optimal meal timing

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Takeaway Messages

• Respect your internal clock: Your body runs on a timing system woven into nearly every cell—disrupting it can have massive effects, from your mood to serious long-term disease risk.
• Eat with your clock: Heavy meals early, light meals late—when you eat matters almost as much as what you eat for metabolic health.
• Modern life challenges: Electrical lighting and late eating patterns are at odds with our biology; small adjustments in schedule can pay major dividends in energy, well-being, and health.

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Transcript

[A]

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 evolution 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, circadian rhythms. You know, the body clock, as it's called. What are they and why do they matter to us?

[B]

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 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.

[A]

Is that right? I never knew that. I assumed it was only the light that caused the flowers to move.

[B]

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, mercifully abbreviated to SCN. And it comprises about 50,000 cells. And when I started in the business, it was assumed that circadian rhythms, 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.

[A]

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 or have I made it too simple?

[B]

No, no, it is the master clock. And 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 it's a bit like the master clock in the brain is a conductor of an orchestra. She is producing a rhythmic tem signal from which the rest of the orchestra, the body, takes its reference cue.

[A]

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?

[B]

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. We do know, for example, that the classic sort of rhythmic change in metabolism that you see is if you cut the nerv 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 messengers what happens.

[A]

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?

[B]

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 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 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, sort of this set 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 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 pilot 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, sound very good. 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.

[A]

Is there a circadian rhythm for our gut? 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?

[B]

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 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 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 in 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, nice lunch, minimum bowl of soup in the evening.

[A]

As you can imagine, hosting this podcast, running Zoe, juggling family life, it all keeps me pretty busy. So I try as best I can to stay energized and show up well in all those parts of my life by fueling my body with the right food, by exercising, and by adding a scoop of daily 30 to my meals every day. If you haven't heard of Daily 30 yet, it's the gut supplement designed by our gut health scientists here at Zoe, it's made of over 30 high quality hand picked plants including seaweed, fungi and different types of fiber. Better yet, it contains ingredients that support gut, health, digestion and energy, which is ideal for packed calendars and busy lives. Simply add one scoop a day to any meal for an extra boost of fibre and plant diversity. And because it tastes delicious on just about anything and adds a satisfying crunch, it quite quickly slots into your life, becoming a daily healthy habit you'll always have time for. By the way, whenever we talk about Daily 30 as a good source of fiber, we're required to say that it contains 4 grams of total fat per serving. Obviously that's all amazing healthy fats from plants, so order yours [e-mail] daily30. Thanks for listening and see you next time.