Published 19th June 2024

The power of mastering your body clock, with Prof. Satchin Panda

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Our bodies work on a roughly 24-hour cycle. It’s called the body clock, circadian clock, or circadian rhythm.

Modern lifestyles keep most of us from living in sync with the circadian rhythm, which puts our health and well-being at risk. 

Eating and sleeping at the right times are really important ways to align with the circadian rhythm and reduce the risk of chronic disease. 

In today's episode, circadian rhythm expert Prof. Satchin Panda tells us how light and food are master regulators of the body clock.

He describes how aligning your lifestyle with your body clock can improve your health, mood, and energy levels — and how to do it. 

Satchin is a world-leading expert in circadian rhythm research. He’s an associate professor at the prestigious Salk Institute for Biological Studies and a recipient of a Dana Foundation award for brain and immune system imaging. Satchin is also the author of two best-selling books, The Circadian Code and The Circadian Diabetes Code.

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Episode transcripts are available here.

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

Today you'll hear from one of the world's foremost scientists on why living in harmony with your body clock is critical for your long-term health. Prof. Satchin Panda is here to reveal why our body clocks govern so much more than just how groggy we are when we wake up. Satchin is a highly cited associate professor at the renowned Salk Institute in California and a best-selling author.

He studies circadian rhythms, their regulation, and their impact on our health. Today, we hear how to harness these powerful, invisible rhythms to increase our resilience to chronic disease.

Satchin, we have a tradition here at ZOE: We always start with a quick-fire round of questions from our listeners, and we have some strict rules. You can give us a yes or a no, or if you absolutely have to, you can give us a one-sentence answer. We know that's hard for professors, but they mainly manage it.

Are you willing to give it a try? 

[00:01:16] Satchin Panda: Sure. 

[00:01:17] Jonathan Wolf: Wonderful. Does every part of our body follow a 24-hour cycle? 

[00:01:21] Satchin Panda: Yes. 

[00:01:22] Jonathan Wolf: Can eating breakfast at the wrong time wreak havoc on my circadian rhythm? 

[00:01:27] Satchin Panda: I'm going to explain what is the wrong time because breakfast is not the morning meal. It's breaking the fast, which has to be consistent.

[00:01:35] Jonathan Wolf: If I live my life out of rhythm with my body clock, can I damage my health? 

[00:01:41] Satchin Panda: Yes. 

[00:01:42] Jonathan Wolf: If I adjust my lifestyle to match my circadian rhythms, can my health improve in just weeks? 

[00:01:47] Satchin Panda: Yes.

[00:01:48] Jonathan Wolf: That wasn't so bad was it? So, now, the final question, and you can have a whole sentence. What's the biggest misconception that you hear about the body clock?

[00:01:57] Satchin Panda: That people have very different types of clock, which is not true. Everybody has the same 24-hours clock.

[00:02:02] Jonathan Wolf: So look Satchin, I think most of the people listening to this podcast have heard of the body clock or circadian rhythms and intuitively they know that if that body clock is disrupted, they don't feel great. I think any of us who have had children know what it's like to suddenly have your sleep broken or suddenly to be on weird cycles.

I think many people like me have had experience of jet lag where you go and move to a country that's a long way west or east, and then it's very difficult for a few days. 

But I was always brought up to understand that it's just really mainly about sort of adjusting your sleep pattern after this transatlantic flight or making sure that, the sort of thing I always say to myself, son which bores him, which is he's a teenager. Oh, you should go to bed at a regular time and wake up at a regular time. That's the sort of thing that my mother used to say to me. 

But I think there's a lot more science that has been happening in recent years. And you're one of the leading researchers in the field. I'd actually love to start, before we dig into how it impacts our health, just to understand a little bit about like, what are circadian rhythms and how do they work?

[00:03:11] Satchin Panda: Yeah, circadian rhythms literally means 24-hours rhythms. So that means these are the internal timetables that are present in every organ, actually in every cell in our body that tells each of our hormones, brain chemicals, digestive juice, enzymes, or even every single gene to turn on and off at a specific time of the day or night.

[00:03:37] Jonathan Wolf: How do you know now that this exists and is in every cell? Is this something that we've known forever? Or is this relatively new.?

[00:03:49] Satchin Panda: Almost 25 years ago, scientists figured this out. They developed a new way to monitor circadian clock or 24-hour clock in individual cells. 

And what is interesting is if I take a cheek swab or my skin cells and then put a magical gene that's from a firefly into it and the cell will glow and dim down in every 24 hours. And that's how scientists came to know that almost every cell in our body, including our skin, our fibroblasts, for example, even hair follicles, all of them have a circadian clock.

[00:04:28] Jonathan Wolf: And does that mean, because you might imagine that maybe the cells are in a 24-hour cycle, but it doesn't really matter somehow for when they get turned into, I don't know, a particular… like your, your heart or your brain or whatever.

Is it true that all the parts of our body are also on these 24-hour cycles? Or, I mean, my heart is working after all, the time, it doesn't switch off, so is that on a 24-hour cycle?

[00:04:56] Satchin Panda: Yeah, actually, the heart beats 24 hours, but what is interesting is, when we sleep, the heart actually slows down slightly. And then anticipating waking up, it begins to beat slightly faster. 

And you might say, well, this is a small change of heartbeat, does it matter? And actually, it does matter because there are many cases when your heart doesn't slow down at night, doesn't rest because just like our brain needs rest, our heart also needs some rest. Those people have a higher risk of heart disease in future. 

So it's pretty well documented for blood pressure also. So for example, our blood pressure dips at night and there are many individuals who have high blood pressure and that blood pressure doesn't dip at night, versus there are some people who have high blood pressure, but that pressure actually dips slightly at night.

And there are a lot of studies showing that non-dipping blood pressure actually has higher risk for heart disease in the future than dipping blood pressure. So you can have high blood pressure, but if still your blood pressure still follows a circadian rhythm that has a less risk for future heart attack.

[00:06:13] Jonathan Wolf: And so, Satchin, both those examples you gave are sort of things that are sort of slowing down or changing during nighttime. And it makes me think about, you know, we're all very familiar with sort of like the sleep mode on our phone or our computer, right? Where there's a sort of energy saving when it's not needed.

Is this 24-hour cycle all about switching things off when I should be asleep? Or do we know why it's there? And is there a simple answer across everything? 

[00:06:44] Satchin Panda: Well, there are many things that go on in our cell, individual cells. For example, our cell has to take nutrition in, produce energy, and during energy production, there is some reactive oxygen species.

So these are the byproducts of metabolism that are quite toxic to the cell. And similarly, there are many chemicals that are also produced during metabolism. You can say toxin or unwanted chemicals, they have to be cleaned up. 

And that cleaning up doesn't happen 24/7, just like your garbage truck doesn't come 24/7. It comes at a specific time of the morning. Similarly, when we sleep, that's the time when actually our body repairs itself, gets rid of the toxin. And it's not that the whole body is slowing down, actually the recycling, the rejuvenation, the repair process actually turned on, during that time

[00:07:39] Jonathan Wolf: Got it. So my analogy was a really poor analogy. It's not that it's going to sleep, it's more like saying, hey, we're switching over. I've heard Tim give this analogy, it's like the night crew is coming in and doing all of this cleaning and work. So actually lots of activity but the point is they can't come in if like the daytime people are doing all their normal activity. 

So it's actually about shifting to a different set of processes inside us. 

[00:08:04] Satchin Panda: Yeah. So that's why if the cleaning crew doesn't come in at night time and repair all the damages, then the next day you'll have a crappy day. You better be as productive at work as you would be if the offices are clean and everything is nice for you to be in.

So the same thing happens in our body. When our circadian rhythms are disrupted, then this repair and rejuvenation cycle is dampened or disrupted. And slowly the body accumulates a lot of these unwanted chemicals. And if it happens in the heart, then that can lead to an increased risk for heart disease. 

If it happens in the gut or the digestive system, then that can lead to digestive issues or even in some cases, in extreme cases cancer of the digestive tract, colon cancer, etc. 

[00:08:54] Jonathan Wolf: I'm sort of curious with the overlap between the circadian rhythm and sleep, because we associate also obviously this cycle at night time with going to sleep, and you're describing it as also this period when all of this rest and repair happens.

But sleep is obviously not something that's in every single cell. How should we think about these two fitting to… are they completely separate? Are they almost the same? 

[00:09:19] Satchin Panda: Well, sleep is a very different state. During our sleep, a lot of things are happening. Our body temperature cools down, core body temperature cools down and actually peripheral skin temperature slightly goes up.

Your digestive system also slows down, almost shuts down. Your breathing slows down. So sleep is not necessarily a function of the brain. It's actually a function of the entire body. Your muscle tone also goes down because you don't want to dreamwalk or sleepwalk. 

[00:09:49] Jonathan Wolf: And so Satchin, I'd love to transition to this topic about how all of this impacts our health as human beings. So I think you've explained the circadian rhythm is there in all of these individual cells. How is that impacting our health as a result?

[00:10:08] Satchin Panda: So let's start with a few things that usually happen in our normal life. One is just like when you have a watch you can anticipate what's going to happen.

So for example, if your office starts at eight o'clock, then around 7.30 or 7.15 you know that the traffic will be 30 minutes. Whether you're taking a bus, train, or driving, it'll take 30 minutes. So at 7.15, you are getting ready.

Just imagine, if you didn't have a clock, then you'd wait till 8 o'clock, close to 8 o'clock, seeing other people driving to work or moving towards work, then you'll start, and then you'll be late to your office, and when you're late, then it doesn't do well on your performance.

So one function of circadian clock is actually, it prepares us for what is going to happen next. So, for example, early in the morning, as I mentioned, one or two hours before we wake up, our sleep hormone, melatonin, is already on decline, so it's making sure that we don't feel sleepy when we are ready to wake up.

We breathe slightly more and then our body temperature begins to rise. All of these preparations are happening, anticipating you wake up. 

[00:11:30] Jonathan Wolf: Which is very different from the way that you feel, isn't it? Because the way it feels, particularly when your alarm clock goes off, is like, Oh, I was in such deep sleep I've got to drag myself out. 

But it's interesting. And I've seen this similarly when you wear a blood sugar sensor. Interestingly, you see this also, I've seen this myself, in my blood sugar, that like, I feel like I'm dragged out of bed but actually, interestingly I've seen that my blood sugar has tended to go up maybe for like an hour or two, I think, beforehand.

So what you're saying is, my body knows a lot that my brain doesn't. So my brain is complaining, but somehow my body already sort of knew where I was going to be. 

[00:12:08] Satchin Panda: Actually, what happens is when you have to wake up to alarm clock, that is a sign that you haven't slept enough. You are waking up one or two hours before your body is supposed to wake up. By the time your body is not prepared, you're trying to drag yourself out of bed. 

Then the question is, so what harm is there? And actually for most of us just imagine if you have a new car, even if you do bad driving, it's not going to affect your performance. But if you have a really old cranky car and you are doing bad driving, it's going to show up. The engine might give up.

This is exactly what happens and we have seen this experiment done over the last hundred years all over the world. When there's a daylight saving time or twice a year the time changes. And at least in the fall when the time actually falls back, so that means you have to wake up an hour earlier than the previous night. That morning, a lot of people, millions of people, are waking up when their body is not ready yet. It's just one hour change and that's the time when we see there is a spike in heart attacks in the morning.

[00:13:26] Jonathan Wolf: There's a spike in heart attacks on the day of daylight saving. That's amazing and terrifying. 

[00:13:36] Satchin Panda: And also that happens to the older individuals, but then those who have a weaker heart. 

There's also the day when there's a spike in road accidents in the morning. Because even young adults and middle-aged adults who are driving, their brain is not ready enough to do that very complicated calculations.

Because when you're driving, your brain is doing a lot of calculations. At what speed you're going, you're guessing what's the speed of the car in front of you on the side, when you're changing lanes. 

[00:14:03] Jonathan Wolf: And you really see that in like the total statistics in the States, there are like more road accidents the following day than across all the other days?

[00:14:12] Satchin Panda: Yeah. And also, if you just look at heart attack incidences that are reported to emergency room, this is again, data from the last 30, 40 years, showing that even on a daily basis, there is a spike in heart attacks early in the morning. 

Because that's when the heart has to start pumping slightly faster than in the nighttime. And this is when there is a higher incidence of heart attack. 

[00:14:38] Jonathan Wolf: So you're giving that as an example where if your body had time to prepare, then it's less likely. But because suddenly it's like, I'm getting up, I need to walk around, and presumably these are people whose hearts are not in very good shape, it's like too big a shock. 

And you're saying there was a heart attack. Whereas if this body clock had been able to prepare you, then you would have been okay. 

[00:14:56] Satchin Panda: Yeah. So this is an example of anticipation. Your body, your circadian clock is anticipating, waking up and preparing. 

So the other thing is to do two different things at two different separate time of the day, because you cannot do those two things together. Just like you mentioned, like the custodial staff or the cleaning crew comes late at night or very early in the morning, who cleans up the office. 

And you cannot have the cleaning crew coming in when you're in the middle of a meeting during the daytime. 

[00:15:32] Jonathan Wolf: Help us understand, what does that mean? Like, what needs to stop in, order for this other thing to stop that actually matters for our health?

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[00:15:37] Satchin Panda: So almost in every cell, there are many chemical reactions that are going on. 

One type of chemical reaction is what we call oxidative process. So that means this is a process where oxygen is used to typically break down things. For example, when you are breaking down fat and making ketone bodies, these are the small, tiny fat-like molecules which have a lot of benefits.

That's one kind of process, but then the flip side of that process is making fat. When we eat a small part of our nutrient that we eat, is actually used to make fat. And this breaking fat and making fat cannot occur at the same time. And they are temporarily or timely, they are separated to two different times of the day.

Similarly, there are also many other examples where the body is programming our cells, or the circadian clock is programming our cells to do certain things that are safer to do at night. And one example is when should our cells divide? Our skin actually repairs itself, or recycles itself, roughly in every 15 to 30 days.

So that means we have new skin every 15 to 30 days. Similarly, our gut lining also changes in 15 to 20 days.

[00:16:57] Jonathan Wolf: Obviously we talk about gut and gut health quite a lot on this show. There's no sunlight inside. So what's happening to the… why is the gut cells on some sort of 24-hour cycle? 

[00:17:10] Satchin Panda: Well, there are a lot of things that we eat. Those are not actually nutritious. There are many toxins and they also damage our gut lining. And just like you cannot repair a highway when the traffic is still flowing, you cannot repair the gut lining when it's still digesting food. 

So that's why the gut lining also repairs itself at night time. And at night time, the movement of food through the gut, that also slows down. So all of this helps to repair the gut. 

So now imagine if someone is a shift worker, or if you have jet lag, you flew from London to California and you thought this is sunny day, let me go lie on the beach. But actually, your skin cells are dividing because they're still in London time. Unless you have enough sunscreen, you are actually at a high risk for certain damage to your skin.

So, this is another example where the circadian clock not only times things to keep us safe within our body, it also times our activities in a way that it's safe from damaging effect of UV light from the sun. 

And then the third one is pretty obvious, that the compatible processes, things that should happen, that can happen together, actually happen together.

So, for example, we are more active during the daytime and we are more likely to have access to food. So, it's very obvious that most people feel hungry in four to five hours during daytime. And they can eat, and the pancreas is also primed to produce more insulin during our wakeful time than in our sleep time.

Our digestion is also programmed to be more optimum during our wakeful time. So these are the compatible processes that are aligned together.

[00:18:55] Jonathan Wolf:  A lot of people listening saying, yeah, that sounds really good. I'm not sure that my lifestyle is actually that in line with my circadian rhythm, you know, I know that every morning during the week I wake up much earlier than I really want to. And I know that I'm a great example of this. 

I sort of know I'm not supposed to snack after dinner, but I can tell you last night I was watching Netflix at about 10 p.m., thinking about this show as I ate my dark chocolate, because I'm really addicted to it. Thinking that you were going to tell me today that I was naughty. 

So like many of us in our modern life, right, because of electric lights, all these things, you know, we're not living sort of just in line with this cycle. How significant can the impact on our health be? 

[00:19:41] Satchin Panda: There are three or four different lines of studies to figure out what happens if we don't pay attention to circadian rhythms. 

The first one I would say is an epidemiological study or looking at what happens to people who do evening shift or night shift work. Are they at a higher risk for certain disease than people of the same age and gender who are not doing night shift work or evening shift work? 

Nearly one-fifth of the population in many industrialized countries work in shifts. And nearly one-fourth to one-third of the population in developing countries work in shifts. So there are a lot of people. 

[00:20:21] Jonathan Wolf: And how big is that impact on your health if you're doing these night shifts? 

[00:20:25] Satchin Panda: It's very clear that shift workers are at a high risk for metabolic disease, whether it's obesity, diabetes, heart disease, vascular disease, kidney disease, all of these diseases. 

They're also at a high risk for many types of cancer, to the extent that World Health Organization categorized night shift work as a potential carcinogen. It's similar to chemicals that are known to cause cancer. 

[00:20:56] Jonathan Wolf: And Satchin, is it only shift workers who need to worry about this? Or for the rest of us who are lucky enough that they're not having to work in the middle of the night, but they're still probably not following, you know, a real cycle.

I could tell you that I am definitely not going to bed when it goes dark outside. Is there any real risks or not? 

[00:21:16] Satchin Panda: Yeah, we don't have to go to bed right after sunset. There are researchers who have gone to populations which don't have access to electricity or very minimum amount of electricity. 

And they find that these populations still stay awake in the evening. They have a candlelight or firelight. And they go to sleep between 9.30 and 10.30 p.m., and then wake up around, say, 6 a.m. or around dawn. So it's not that we have to fall asleep as soon as it becomes dark. 

But the other point is, well, if you're not a shift worker, should you worry about it? And the answer is, well, you may not be a card-carrying shift worker, but are you living the lifestyle of a shift worker? 

And that brings up the question of, okay, so what is the lifestyle of a shift worker? Or what is typically defined as shift work? There is no internationally recognized rule about what is defined as shift work, but there are many European countries who roughly define shift work as staying awake for two or more hours between 10 p.m. and 5 a.m. and engaged in some physical activity or mental activity. 

It's not that you are lying in the dark room and you cannot sleep for three to four hours. That's not shift work. But you have to be awake and be active and engaged in some kind of work. 

So now just imagine how many people stay awake for two or more hours between 10 p.m. and 5 a.m. A lot of people. What it really means is, are you reducing your sleep by roughly two hours for 50 days in a year? So that is once a week. 

[00:23:01] Jonathan Wolf: So it's pretty easy to achieve, is basically the message you're describing. Like, just the weekend, decided to stay up late, or staying up one night during the week late, or any of these things can have that impact very fast. 

I think you've painted the picture about why this is really important and that there's real health risks if you're not there.

I guess I'd love to talk about, okay the practical tips for somebody listening to this about ways that they can adjust their lifestyle. And I'm particularly interested in food and sleep that our listeners could actually take. 

And I think I'd love to start with time-restricted eating. And that is in part because I have to admit, I was very skeptical about this just a couple of years ago. And when I first heard about it, it sounded pretty crazy. It was like, oh, you know, it's another one of these fads that somebody's come up with and just drinking cauliflower soup. It doesn't make any sense. 

But actually, ZOE did a huge study last year which was called the Big IF Study with around 37,000 people. Because it's something Tim and Sarah were really interested in because I think of research like your own.

And what was really striking was, you know, on this very large sample of people, there was a real impact on energy and hunger after just a few weeks of making this adjustment. Which I was really surprised by. So I've had to accept there's definitely something around this. 

So could you tell us about, based on your own research and the other researchers out there, what does all this circadian rhythm tell us about when we should eat? 

[00:24:42] Satchin Panda: So there are two fundamental discoveries about what resets or synchronizes our circadian rhythms.

One is light, the daylight or light that is rich in blue light that goes through our eyes and resets our brain clock and resets when we sleep, when we wake up. 

So that's light is one cue, but the other cue is when we eat. If we eat at the wrong time like one night if you're eating very late into the night, then that night your body clock gets confused thinking Huh? Was it a delayed dinner? Or was it an early breakfast? 

And for the next few days, it gets confused. So all the body clocks, the clocks in the liver, gut, heart, kidney, all of those clocks get disrupted. So they can't work pretty well. 

[00:25:36] Jonathan Wolf: So just a late night can like mess up all, and it's not even just my gut.

It can even mess up my liver or my heart. That doesn't sound good, as someone who has a tendency to maybe have that little snack before bedtime. 

[00:25:49] Satchin Panda: Yeah, because your body is thinking, well, was it evening, was it a late dinner? 

So, after we learned this. And the question was, well, we know that having a strong circadian clock is good for health. In modern life, it's very hard to control light exposure because from morning till night and also late into the night until we close our eyes, we're always exposed to light. It's very difficult to control that. 

What can we control? When we eat, and just by when we eat, can we change health? So what is now popular as time-restricted eating or intermittent, very popular form of intermittent fasting, it was not there 12 years ago.

So, my lab was the first one to do this experiment in mice, just like all basic scientists do, because we can control the experiment pretty well, and we can also do molecular analysis. 

So, we took a few groups of young mice, and they were identical in genes. They were born from the same parents, grew up in the same room, they had the same microbiome, they ate the same number of calories from the same food.

The food we gave them was relatively high in fat and carbohydrate, which would be equivalent to eating, say, fish and chips every day, or a burger every day. You know, all the bad food you can think of. 

[00:27:17] Jonathan Wolf:  Not the healthiest diet for a mouse or, or a human being. 

[00:27:21] Satchin Panda: We knew that when mice are given this kind of food, and they're allowed to eat whatever they want to eat, they become very obese, diabetic, they have high risk for cardiovascular disease, and also the increased risk of liver disease and cancer.

This is from 10,000 experiments that were done before we did the experiment. And they get the same number of calories also, but the first group was allowed to eat whenever they want, and the second group was allowed to eat within an eight-hour window, during nighttime when they're supposed to eat. Mice are night active, so we give the food at night time.

And within three days these mice will learn that the food is available only for eight hours. So they eat the same number of calories within that eight hours, as the mice that had access to food 24 hours. 

It's a very simple experiment. We monitored their food every single week to make sure that both groups are eating the same number of calories and we weighed them every single week. 

And at the end of 18 weeks there was this huge surprise that the first group of mice that ate around the clock as expected, they were obese, diabetic, they had high cholesterol, liver disease, all the bad things happened to them.

While the second group that ate within eight hours, surprisingly, miraculously, they were completely healthy. 

[00:28:44] Jonathan Wolf: Which is crazy, right? You're saying they ate the same amount of food, the same type of food, and just by restricting their eating to eight hours, they managed to deal with this unhealthy diet. Whereas the ones that could graze whenever, ended up getting really obese.

[00:29:00] Satchin Panda: We repeated this experiment with three independent researchers in the lab who did not even overlap with each other. To make sure that this is true. 

Because think about it over the last 150 years of nutrition research, we know that what we eat is important because we have to eat a healthy diet. There is no doubt about it. 

How much we eat is also important because there is a rich literature of almost 100 years showing that reducing food intake improves health, and in many laboratory animals at least, it increases longevity. 

[00:29:37] Jonathan Wolf: And I think this is a fascinating discovery, and you should feel really proud. 

Satchin, I would love to take this now, you know, for our listeners, who's like, you've painted this incredibly powerful picture of the mice. But of course, none of our listeners are mice. I don't think any of our listeners are mice. 

So if you're a human being listening to this, and a number of our listeners will also have heard that mice studies don't always translate fully to human beings.

What is the evidence say, today? What would your advice be for someone who's saying, well, what does that mean about how long should I be not eating for during each day? 

[00:30:17] Satchin Panda: Okay, so the first thing is what is actually considered food? Because a long time even for my mouse study, I was not getting funding. So normal criticism I would get is, well, people eat three square meals within 10 to 12 hours, so your findings have no human health implications. 

So then we started looking into when people actually eat. And we know that we eat at different times on different days, and sometimes just like we wake up late in the weekend, those who are working a normal day shift, they also wake up slightly later in the weekend. 

We also change our breakfast time. We change our dinner time. And to our surprise, until 2014, 2015, there was not even a single study that objectively showed when people eat from one day to another for a complete one week. And we thought that, well, we have to do this experiment.

So we had 156 people doing this for three weeks. And we said, just take picture of everything, even a glass of water, whatever you're doing. So we went through the pictures to make sure that we discount all the water and low-calorie, noncaloric food. And what we found was less than 10% of people actually eat or drink all the energy-containing food and beverages within 12 hours or less.

[00:31:39] Jonathan Wolf: Less than 10% in your, in your study. 

[00:31:42] Satchin Panda: Yeah. And these people, none of them was a shift worker, and these are all nine to five regular job people who do regular jobs or who are homemaker at home. 

And at the beginning of the study, we had actually asked them, when do you think you eat? And almost 90% of them had answered that they eat all that food within 12 hours.

So here's a disconnect between when people think they're eating versus when they're actually eating. 

[00:32:10] Jonathan Wolf: We have about a hundred and fifty thousand people who are ZOE members now, and you know, it's part of this they're logging their food. And we similarly see, firstly, there's a lot of variability as you say, and secondly that there's a lot of activity generally in the evening.

Because it might be I think I've finished my food, but then maybe I can have a glass of wine later in the evening, for example. Or, you know, I'm gonna have a snack that I don't really think about because that's just like a snack. It's not planned. 

So what is your best view today about if someone's listening to this and saying, you know what, I'm currently not eating all my food within even 12 hours. Satchin’s saying this is really bad. What do you feel the data suggests that at the moment that someone should be thinking about as the right way to manage the duration when they should be eating and then fasting?

[00:33:00] Satchin Panda: Yeah, so what I say is almost anyone from a 10-year-old to a 100-year-old can and should eat all their food within 12 hours. 

So whether you are healthy, unhealthy, you should be eating within 12 hours. The reason is, even after your last meal, your stomach and the digestive system takes another five hours to digest that food completely, absorb all the nutrients.

So although your mouth finished doing its job, your stomach is still working for an extra four to five hours. So only after four to five hours, it's getting a little bit of rest. So even if you are eating for 12 hours, your body is getting only seven hours of rest from food. 

Then the question is, well, if someone has high blood pressure or pre-diabetes, for example, or moderately high level of cholesterol, of many of these risk factors, then what can they do?

So this is where it becomes a little bit difficult because those who are headstrong and can eat everything within, say, six, eight, or nine hours, then they can do and they might see a lot of benefits. 

And so far, what we are seeing is there are many studies, not only from our lab, from many other labs, when people reduce their eating window by three or more hours. So this is where it becomes a little tricky. 

So for example if you are consistently eating from say six o'clock you get up and then by six thirty you have your tea with cream and sugar or a biscuit or something else. And that before going to bed at nine o'clock or eight o'clock you have some snack, then it's already pretty big. 

So if you reduce your eating window by three or more hours, and it's not less than eight hours, so then you'll actually begin to see a lot of benefits.

First thing is, many people who do this, they say that within a couple of weeks, their sleep improves. They can sleep much better at night, and the next day, they feel more energetic. 

And then those who have a high blood sugar, blood pressure, or cholesterol, over three to six months, they do see improvement in at least one of these parameters.

[00:35:14] Jonathan Wolf: And Satchin, I've met some people who are saying, hey, I'm doing time-restricted eating, it's fantastic. Therefore, I can sort of eat what I want within this period, as long as I'm restricting the time, that's the key. 

Now, it's not going to surprise you that as we're pretty skeptical about this, but what's your view? Can you solve your health problem with the time-restricted eating without changing what you eat? 

[00:35:40] Satchin Panda: What we find is the people actually do time-restricted eating or intermittent fasting and at the same time they improve their nutrition in an interesting way. So let's begin with what happens in real life.

So those who are trying to do 8 hours time-restricted eating or 10 hours time-restricted eating, they're going through 16 hours or 14 hours of fast overnight. And after this long fast, when they're breaking the fast, they tend to eat a bigger breakfast because they are so hungry after 16 hours of fasting. 

And typically breakfast is, for many people, not for all, breakfast is the healthiest meal of their day because that's when they have complete control of the food, they're at home, they can control what they eat.

And since most of us know that healthy food is good, so a healthy breakfast begins their day. And what we see is they also reduce snacking. So there is less snacking between breakfast and lunch, and also between lunch and dinner. So in that way, they reduce their food intake from snacks, which are mostly not so nutritious food. Even if you are eating a protein bar that has a lot of sugar. 

Then what happens is those who are finishing their eating before 8 p.m., whether 5, 6, 7, or 8. And since kitchen closed, the bar also closed, so they also reduced their alcohol intake. 

And this is what we specifically saw in one study with firefighter study. We saw that those who reported drinking alcohol at baseline at the beginning of the study, they significantly reduced their alcohol intake if they were in the time-restricted eating group. 

So these are different ways people actually inadvertently improve their nutrition. 

[00:37:27] Jonathan Wolf: So you're saying often they combine the time-restricted eating actually not with making their diet worse, but making their diet better in part because it's like, well, I'm really hungry and I've got control because I've got my breakfast and also because I'm going to not eat a lot of these like bad snacks at bad times or whatever.

I know that this is still an area of very early research and it's something where ZOE is actually very involved as well because it's so interesting. 

Right now,  you mentioned that almost everybody should be trying to restrict their eating to within 12 hours. What is your view about the added health benefit of restricting that to 10 or, or eight?

[00:38:08] Satchin Panda: So those who are current eating habit is 12 hours, if they reduce it to eight hours, they will see a benefit. And those who are, whose current eating habits spreads over 14 or longer hours, if they reduce it to 10 hours, we also see health benefits. 

So the bottom line is if you can do eight hours, it's better. Ten hours is not that bad and 12 hours you should do in maintenance more. Because once you're healthy and if you're athletic, if you're doing a lot of physical activity. Or for example those who had a baby recently, who cannot fast for a very long time for obvious reasons, they can still try to eat within 12 hours.

So that's why I say there is no single hard and fast rule.

[00:38:50] Jonathan Wolf: Brilliant. Well, thank you for answering that. I have one final question, which we get a lot. Is drinking tea or coffee outside of this window sort of breaking it or acceptable? 

[00:39:05] Satchin Panda: Yeah, so tea or coffee, in one of the studies with firefighters, we actually allowed them to have black coffee and tea outside the window because they’re finishing their shift after 24 hours in the mornings. So we want them to be alert and awake when they're driving home or in the middle of the night when they're responding to a fire.

There are a few rules or guidelines for tea and coffee because tea and coffee will affect your sleep. And we know that almost all of us take at least six to eight hours to break down the caffeine, the active ingredient that keeps us awake, by 50%. 

So that means if you had one cup of coffee at noon, you still have half a cup of coffee or tea at 6 p.m. in your system, and quarter cup of coffee or tea at midnight in your system. 

So that means to improve your sleep, it's better to stop coffee or tea by around 2 o'clock in the afternoon. So, I say no coffee or tea after 2 p.m. if you’re planning to go to sleep between say 10 p.m. and midnight.

Because another thing is you need a good night's sleep, because when you can't sleep enough, or when you're not sleeping enough, then your brain actually craves for unhealthy food. 

[00:40:26] Jonathan Wolf: And what about in the morning Satchin? I think a lot of people, and I'm included here, have found one of the ways that you can push out your breakfast time is that you have, I tend to, I like tea. I tend to have a black tea. Other people might have black coffee in the morning when you wake up and then eat quite a bit later. 

Is that breaking any of these magical things you're describing? Or do you feel quite good? Or are you going to say the experimental data isn't in yet? 

[00:40:54] Satchin Panda: Actually, in many of the studies that are published, the participants were allowed to drink black tea or black coffee. Having said that, if you're drinking like one liter of coffee before your breakfast at noon, then be mindful about that because having too much of hot coffee or tea in an empty stomach for some people, not for all, for some people it can increase the risk for acid reflux or heartburn.

So if you begin to see that you are having acid reflux or heartburn, and you have this habit of having morning coffee or tea to delay your breakfast time, then maybe that's not the best practice. 

[00:41:32] Jonathan Wolf: I would like to try and do a quick summary of the conversation if that's all right, and please just correct me if I get any part of this wrong.

We started by your explaining that every cell in my body has this 24-hour circadian rhythm. And so that's sort of amazing in every part of me. And that that then leads to all the different organs in my body, sort of making these different decisions based upon the time of day. 

So you described that at night my blood pressure goes lower, my heart rate might go a bit lower. But then before I wake up, it all starts to reverse so that I'm ready to go. So like my whole body is changing and getting prepared for what's coming next. 

You gave this great example of how there's this experiment once a year where daylight saving happens and everybody wakes up an hour early and there's this big spike in the number of people having heart attacks, the number of people having road accidents. Which just goes to show how important this circadian rhythm is if we don't live within, even just by one hour you have a real impact.

And I think you then explained that these clocks do a lot of different things but I took away that there's maybe two really critical things.

One is sort of preparing us for what's going to happen next. So this is making sure that in that example, you don't have a heart attack because your heart has already got things going. And so you can see that if you're living out of rhythm, on Saturday morning, I'm having this completely different pattern than on Friday, that you're in all sorts of trouble.

But the other part that you talked about is, there's a lot of processes in our body that need to happen when another process stops. And you talked about the cleaning crew, which is a wonderful metaphor. So, you know, your gut can't heal itself if there's food going through it. So it needs to know that at nighttime, you're going to stop eating, it's going to fix it.

But you also gave this great example of your skin, which actually grows at nighttime so that the sun doesn't come on it when it's fresh. 

So all of these things that are designed to be in a cycle, driven by a world where, you were saying, as human beings, we might stay up for a few hours after dark to talk around the fire, but then we go to bed.

So our whole body is built for this system, and then in the last 50 years, we suddenly introduced this cheap electric light, and everything has changed. And suddenly, you can sleep through, you can also have your alarm go off. much earlier than you want to wake up, and so all of these things have changed.

That's not good. So how do you get these things aligned? And you said there's two main ways to sort of fix your circadian rhythm. One is with light exposure. We said that's actually quite hard to manage in our life because the electric lights, I'm sitting here right now, it's already gone dark outside, the lights are on.

You said, interestingly, the way that you eat can reset your clock. And this is, I think is fascinating new science. So if you're eating at the wrong times, your body clock gets really confused. So if I eat this really late snack, my body's like, well, hang on a minute, is that breakfast or the morning?

But if I can eat at the right time and give myself these long breaks, I could actually start to live with my circadian rhythm. 

You described this famous study that you carried out, which I know is incredibly well regarded, where you said, look, I've taken these identical mice, I'm letting some of them eat whenever they want, I'm letting the others eat only for eight hours.

And even though you gave them this terrible diet, that's a bit like the standard American diet, actually, the mice that were just eating eight hours a day were able to cope with it. Whereas the ones that were eating all the time, they had all of these health problems. 

And then more recently you've described these new studies starting to look at what this means with humans, and it's relatively early, but I think what you're saying, which is really interesting, is very few people are actually eating even within 12 hours.

So most of us in the West are eating more than 12 hours. And you're saying, you know, Almost everybody should be able to eat within 12 hours. You said from 10 years old to 99, you should be able to.

So if you start eating at 9 a.m., you should finish at 9 p.m.. And that's quite strict. You can't have a glass of milk or a glass of wine or even a chocolate biscuit. You've got to not eat, and that's to create all of this time for cleaning,

And then I think you said, look if you can manage that, actually there can be even more benefits if you can focus further restrict the time. And in your studies generally you're restricting people's eating to between eight hours and ten hours. And if that means there's a significant reduction from what people were doing before you've seen real benefits, particularly quite rapidly things like better sleep and better energy and which will be very similar to some of the results from our own studies.

And then over longer time periods, sort of three to six months, you're seeing impacts on some of these clinically important measures like blood pressure, and they're like, you didn't want to overstate how big they are, so statistically significant, not transforming. 

And then I think the key message that I think you get left was you can't just eat rubbish food and do this and expect that this will fix everything. Actually the good news though is that by doing this you might actually find it easier to adapt a healthier diet because you potentially have more control, eat more over things like breakfast that you have control.

And that you permit tea and coffee outside of the period, but I would say you weren't massively keen on it, Satchin, if I play that back. You were certainly concerned about caffeine, but I would say in general, I would say you're sort of suggesting that I ought to be able to manage even tea and coffee mainly within this period. 

[00:47:00] Satchin Panda: I have to say that if you have this acid reflux, heartburn, that kind of issue, then pay attention to whether having that black coffee in the morning is triggering it or increasing the severity. If so, that may not be the best thing for your gut. 

[00:47:18] Jonathan Wolf: That's great advice. Interesting, you know, we see that also as one of the things that seems to be quite linked to poor diet, unhealthy microbiome. So I feel like in a sense that there's a positive story as well. 

I know one of our scientists, Will Bulsiewicz, talks about this quite a bit, that by shifting your gut health, that's actually one of the symptoms that often you can see that, that potentially reduces. It's interesting how these things are, are interrelated. 

Satchin, thank you so much for coming. I thought that was fascinating. We did not manage to discuss sleep. So I hope that I can tempt you back in the future and we could do a podcast specifically focused on sleep. And I think of course, update probably on the science on time-restricted eating, which I feel is moving sort of very rapidly at the moment.

[00:48:02] Satchin Panda: Yeah. Thank you so much and have a perfect circadian day. 

[00:48:06] Jonathan Wolf: I will do my best. Thank you very much. 

I think it's been pretty incredible to learn from a world-leading expert today about how food impacts our circadian rhythms. I hope Satchin's insights left you with plenty of practical tips to apply to your life.

And it's clear that good nutrition includes eating at the right time. But of course, what you eat is even more important, in terms of its impact on your health. A ZOE membership helps you make the smartest food choices to transform your health using science that's backed by real clinical studies. A ZOE membership could help you feel better now and enjoy many more healthy years in the future.

You can learn more about a ZOE membership and get 10% off by going to 

I'm your host, Jonathan Wolf. ZOE Science & Nutrition is produced by Yella Hewings-Martin, Richard Willan, and Sam Durham. As always, the ZOE Science Nutrition podcast is not medical advice. If you have any medical concerns, please consult your doctor.

See you next time.

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