The Diary Of A CEO with Steven Bartlett - Most Replayed Moment: The Link Between Weight Gain and Sleep! Are Sleep Trackers Harmful Or Helpful?

Episode Date: May 15, 2026

Dr Guy Leschziner is a consultant neurologist, sleep physician, and author who specialises in the science of sleep, circadian rhythms, and the hidden ways our brains shape our nights. In today’s moment, he unpacks how poor sleep can change everything from your appetite and weight to your emotions, cravings, and long-term health. He reveals what your body clock is really doing, and his true opinion on sleep trackers: harmful or helpful? Listen to the full episode here! Spotify: https://g2ul0.app.link/6btyk6znG2b Apple: https://g2ul0.app.link/ZFC5hNCnG2b Watch the Episodes On YouTube: https://www.youtube.com/c/%20TheDiaryOfACEO/videos Follow Guy: https://www.guyleschziner.com/

A couple of weeks ago, I was traveling through Ireland with my team, and I was telling them how I don't love when things in my life sit idle, whether it's my time, my energy, my health, and my investments. If something has value, it should be working, even if just quietly in the background. And one of the most overlooked examples of this was when you're traveling and you're away from your home. Because when you're not in your home, they just sit empty, and they're not doing anything for you. Which is easy not to think about, but it's still a choice that you're making. We're choosing not to get anything back from something that has real, very very. value because our home can easily play a part in someone else's holiday experience. Airbnb is one of my partners, as you know, and hosting with them is a quick, easy way of changing that. You make your

home available for dates that suit you, and instead of it just sitting there, someone else gets to experience and enjoy your home and your neighborhood and your city. Hosting on Airbnb, it also lets you make a little bit of extra money on the side, which you can put towards your next holiday. Your home might be worth more than you think, and you can find out how much it's worth. At B&B.ca slash host. You mentioned earlier that when people don't get enough sleep, the chance of obesity and weight gain increases. How does that happen?

What's the mechanism? So there are probably many mechanisms, but one of the ones that is best understood is that we know that when you're sleep deprived or indeed when your sleep is disrupted by anything else like, for example, sleep apnea, there are changes that occur in terms of hormone levels of hormones that regulate our appetite and our satiety. And so even a single night of sleep deprivation can result in a dramatic increase in your calorie intake overnight. There have been some studies done, for example, in nurses. So there was a study done that followed up nurses for 18 years, and they looked at their weight and how much they slept on a regular

basis. And what they found is that those nurses that were sleeping less than about six hours a night on a regular basis, first of all, started off at the beginning of that 18-year period at a slightly higher weight. But over the course of those 16 years, 18 years, they put on much more weight than other groups. So there is a very clear correlation between sleep duration, sleep quality and weight gain. We see that, for example, in Indian, and in India, who we treat with sleep apnea. So one of the treatment for sleep apnea is a mask-like device that you wear that stops your airway from closing down at night. And for some very overweight individuals, actually, when you treat their sleep apnea, they do manage to successfully lose weight where in the past, they found it absolutely impossible to do so. Okay, so if I'm underslept, I'm more likely to eat more calories the next day. Yes. Am I also more likely to reach for foods that are like high in sugar and bad for me?

Certainly some studies do suggest that. I think it's also important to say that sleep disruption or sleep deprivation has some fundamental effects, for example, on your glucose tolerance, so your insulin resistance, which of course is a particular issue for, for people with diabetes, but it affects us all. So if you're very sleep deprived, there are changes to the way that not only your appetite or what you're reaching for, but also how your body processes the breakdown of those food stuffs. Interesting. Because I anecdotally, I think I can clearly say that

if I'm underslept, I'm much more likely to eat something that is high in sugar or... Well, I think it's not good for me. We can all testify to that, can we? I think everybody has known that situation where they're very sleep deprived and they think, oh, well, I just need a bit of chocolate or, you know. What's going on in the brain, though? Why is that?

Is it something to do with the amygdala and the prefrontal cortex, the emotional center of our brain? Yeah, I don't think we know. I think it's probably to do with the reward mechanisms that underlie our behaviors, that there is something about sleep deprivation that alters the, rewards that we're seeking. But I don't think I can give you a clear answer on that. You also talked about circadian rhythms. If I was a 10-year-old, what do I need to understand about the circadian rhythm, what it is, what it does, and why it's important? So within pretty much every cell of our bodies, there is this 24-hour clock. And in fact, if you take a single cell and

stick it in a petri dish, about 40% of the genes within that cell will exhibit this sort of 24-hour cycle. And that 24-hour cycle really controls pretty much every biological system within our bodies, be it how our liver works, how our heart works, how our lungs work. There is one particular area of the brain called the supra-chismatic nucleus that is viewed as the master clock. It's the clock that coordinates all the other clocks within our bodies. And that influences not only all of these other clocks that are occurring within the cells throughout our bodies, but influences our behaviour as well. So influences, generally speaking, when we feel tired and when we want to go to bed and when we wake up. And also influences things like, you know, when we feel most mentally

alert, when we want to eat, when we want to drink, when we feel most able to cope with work, for example. Now, that circadian rhythm, that circadian clock for most people, confers the sleep onset of somewhere between 10pm and midnight, if you're an adult, and waking up somewhere between 6 and 8am. Now, the timing of that body clock is governed by two things. It's governed by our genetics, so whether or not we are genetically predetermined to be slightly later in terms of our body clock or slightly earlier. And we see that in families where lots of people with, for example,

will say, well, you know, I've always gone to bed late and woken up late, but so is my father, so as my grandfather, et cetera, et cetera. But it's also influenced by what's happening in our environment. About 50% of the definers of our circadian clock are governed by what's going on around us. Be that in terms of when we're exposed to light. So we know that light is a very important driver of our circadian rhythm when we're eating, when we're exercising, when we're doing a whole range of other activities. And also one of the markers of our circadian rhythm is the secretion of melatonin. So there's a very small gland in the center of

our brains called the pineal gland, which secretes a hormone called melatonin. That hormone tends to start being secreted in most people at around six o'clock in the evening. It peaks at the time that we want to go to sleep. And then it starts dropping down a few hours before we wake up. It's almost a a chemical marker of our circadian rhythm. But we also know that giving people melatonin in tablet form, for example, can influence our circadian rhythm. So there's this sort of feedback loop between our own body's secretion of melatonin and what our supraclysmatic nucleus, our master clock is doing. So by giving people melatonin at particular times of the day, we can shift your circadian rhythm forward or back. So that master clock in the brain, where is it positioned? The supercuitary

Pochasmatic nucleus is in a small area of the brain called the hypothalamus. And is that linked to the eye? There are links from the back of the eye to the hypothalamus. So there are some cells in the back of the eye, in the retina, that are not involved in vision, not involved in conscious vision. But what they are involved with is detecting blue light in particular, which is the part of the spectrum of light that is most important in terms of regulating our circadian clock. and there are direct links between these cells that are called retinal ganglion cells and the supra-chismatic nucleus.

So exposure to light, to blue light in particular, is really very important in reinforcing or adjusting our circadian rhythm. Now, of course, we live in a world whereby the seasons change and the amounts of light that we're exposed to changes. So if we were on a set rhythm all the time that was immovable and unadjust, then there would be times where our circadian rhythm might not be, might be at odds with our environment.

So there does need to be some slight adjustment of that circadian rhythm. And light is probably the most important adjuster. Blue light, is that the light that comes from my smartphone? So it's, so blue light, obviously the strongest source of blue light is sun. Yeah. But yes, these kinds of devices, your smartphone, your iPad in front of you, a range of electronic devices also have blue light within them. So that's why if I'm up on my phone until 1am in the morning,

staring into the screen, I'm kind of tricking that sort of optic nerve, which is then impacting my master clock, and I'm telling it that it's a different time. Yeah, so the thinking on that has changed a little bit over the last few years. So, you know, it was said that using these devices will trigger insomnia as a result of blue light. Actually, it's likely that the amounts of blue light

that these devices put out is probably insufficient to do that directly. But there are two effects of using your gadget until 1am. The first is on a long-term basis, if you're doing that regularly, it will result in an adjustment of your circadian clock and push it back. So you will want to go to bed a little bit later

and wake up a little bit later, which is fine if you're self-employed or you don't have any restrictions on your time, but most of us need to be up at a certain time in the morning. And if your circadian rhythm is delayed significantly, the net effect of that is that you're going to end up sleep deprived. I mean, there's also the issue of being, you know, on Twitter or being engrossed in a movie on Netflix at 1am

that is going to make you delay sleep anyway. So there are those two effects. It's arousing. It's arousing, and it grips your attention. Well, what do you make of these people? me being one of them, but watch, you know, serial killer movies when we fall asleep. Well, I think that that's only an issue if it's stopping you from falling asleep. Do you see, because I have this argument a lot with my partner.

She can't understand why I need to watch this, like, really stimulating stuff when I fall asleep. She likes quiet, and I need to, I tell myself I need to, like, listen to something. Do you see, like, variance in this kind of thing, and is there a better approach? This comes down to an individual. individualised approach to sleep. And that's why I'm always very reluctant to say this is the perfect night's sleep. These are the rules for sleep. You know, for example, there are some people who, for whom caffeine doesn't really influence their sleep and can have a double espresso an hour before bed and still have no problems getting off to sleep. For most of us, that's not the case. If you are somebody who can

watch something very stimulating, very scary and then switch it off and roll over and go off to sleep, then I guess that's not a problem for you. For most people, that's probably not the best thing to be doing at night. I think that long term, one of the issues is with all of these kinds of activities in bed at night before you go off to sleep, is that they weaken the psychological associations between bed and sleep. And so if you start associating bed being a place where you're mentally active, where you're engaged, than if you have an underlying predisposition to insomnia, for example, then that can sometimes set the stage for developing insomnia later on.

The average person that you've treated, worked with in your clinical practice that's struggling with sleep, is at the heart of the issue, just poor sort of sleep hygiene, like you've said there. Because I've got so many friends that say to me that they struggle with their sleep. Many of them have struggled with it for years. and I doubt there's some sort of genetic reason why this many people are struggling with sleep. So I imagine it's just some kind of behavioural reason. So I think that the genes that predisposed to insomnia are pretty widespread, but obviously, you know, in pretty much all areas of medicine,

there is an interaction between genetics and environment. And certainly poor sleep hygiene, and that's a horrible term, I hate that term, but it's the term that is most widely used and understood, can certainly put in place certain aspects of behaviour that then can give rise to chronic insomnia in the long term. So if you've got very bad chronic insomnia, then suddenly putting good sleep hygiene in place, it's unlikely to fix it. But it may be that that poor sleep hygiene in the first instance gave rise, or at least predisposed you to developing insomnia. And what is poor sleep hygiene? If I wanted to be the worst possible sleeper in the world, what would I have to do?

So I think you would probably have to set up your home office in your bedroom. Okay. You'd have to have your TV on in your bedroom all the time. Be surrounded by electronic devices. Drink a lot of coffee late in the evening. Alcohol? Drinks a little bit of alcohol. So alcohol in the short term, of course, is quite sedating.

It's a central nervous system depressant, but it does dramatically worsen the quality of your sleep. And for various reasons, the direct chemical effect, the fact that you've got a full bladder, the fact that you're probably snoring a little bit more. So alcohol's not a good thing. You know, not having a wind-down period. So, you know, gambling on the stock market until 1am, switching your laptop and then trying to go to bed, those kinds of things. So that's, you know, the quintessential.

very, very bad sleep hygiene. What about when I eat? So, you know, eating is perhaps less important, but avoiding a very large carbohydrate meal, carbohydrate-rich meal before you go to bed. For two reasons. One is that we know that it can cause some fluctuations in terms of your blood sugar, and also if you've got a bit of reflux, it can make that much worse. Okay.

What about sleeping in bed with somebody else? Well, I think that for some people, they've, and again, this goes back to no one rule for everybody. You know, if you've got a sleep trait termed sleep reactivity, which is where your sleep is very liable to your environment, then obviously sleeping next to somebody who's snoring loudly or who gets up in the middle of the night two or three times to urinate can be very disruptive to your sleep. If you've got very little sleep reactivity, you may actually find it comfortable, more comfortable to sleep with somebody in the same bed as you. I was thinking the the day because where I've currently moved into there's no blinds or curtains in my bedroom

and I was wondering if that might be a good thing because it at least means that in terms of my circadian rhythm I'm waking up at the same time every day because I'm waking up when the sun comes up yeah but the sun comes up at different times on different days firstly so if you were doing that routinely you might find yourself really rather sleep deprived in the summer months and also there is some emerging evidence that exposure to light at night in your sleep is not very good for you. So there was a very recent study that implied that light exposure at night increases your risk of diabetes. So it certainly is not good for the quality of your sleep and the likelihood is that you won't

wake up as soon as it's light, you'll wake up an hour or so after it's got light, but during that hour or so it may have had a negative impact on the quality of your sleep. So having a dark bedroom is really part of good sleep hygiene, as it's having a quiet bedroom that is not too hot or too cold. What if you wear a sleep mask? Does that solve it? Yes, it does. I mean, I wear a sleep mask.

I think it's, you know, particularly if you don't have good blackout curtains or blind, in your bedroom using a sleep mask, particularly in the summer months, is probably very helpful indeed. Does that mean that the only light receptors we have are behind our eyes? Well, I think that there were some rumours on the internet that there were light receptors elsewhere, but certainly the only ones that we know to be of significance in terms of defining our circadian rhythm are the ones in our retinas. Because I did psychology when I was in secondary school, and I remember reading one of of the psychology books, maybe my psychology teacher told me this, I think it was Mrs. Launy,

that there were some studies where they shined a light like underneath someone's knees. Yes. And people would like wake up when they shine the lights there. I think that that's been, that's been very much discounted now. It is a bit strange, though, that the only light receptor would be behind the eyes. It just feels like, I don't know, feels like poor design. That there's only one place where we figure out if it's night or day. So, you know, you see that, for example, in people who've lost their sight.

So in some individuals who have lost their sight completely, then they lose the regulation of their circadian rhythm. And they develop circadian rhythm disorders, something called a non-24-hour rhythm disorder, whereby their circadian rhythm is on, for example, a 25-hour cycle. And so every night they will go to bed one hour later than they did the previous night and will go right the way around the clock, you know, every month or so. So, you know, that's a very good piece of evidence that actually it's those cells in the retina that are of crucial importance for maintaining the stability of our circadian rhythm.

I wrote that down. I wrote down in one recent study, 40% of totally blind individuals had a non-24-hour circadian rhythm, which really kind of for me also highlights just how important it is to think about my circadian rhythm and how much impact it's having on my whole body. The other part you mentioned was the biological element to our circadian rhythms,

which people refer to as chronotypes. Yes. And this idea of chronotypes, which is essentially, from my understanding, that genetically we will have a slight sort of disposition to sleep at certain times. Yes. And to wake up at certain times. Yeah. So there's these names, right?

There's like the owl, the lark or whatever else. I think I'm an owl, but that also could just be bad habits. Yeah. So, you know, certainly there's that sort of genetic. predisposition. There's also these environmental factors, but there's also age in that our circadian rhythm changes a little bit as we go through different stages of life. So, you know, it's not at all uncommon for teenagers to become more evening chronotypes. And then as we get older, we tend to shift back to the morning. So that's part and parcel of our aging biology as well. So I think that,

you know, genetics is really important, but so are other factors. as well. Studies and twins suggest that up to 50% of our chronotype is under genetic control. So does that suggest that we are likely to have a similar chronotype to our family?

We are likely to, but obviously our environment is not going to be exactly the same as our parents. You know, what we do, our behaviours are not going to be identical to our parents or our siblings. So yes, we more frequently see that, for example, people People have a evening chronotype that runs in families.

But that doesn't mean that, you know, look, we're more than just the destiny of the genes that we hold. Obviously, our genes are really important, but so are other factors as well. If someone came to you and they said, I'm struggling with my sleep, you know, multiple nights in a row I haven't been able to sleep. I'm getting in bed and nothing's happening and then I'm waking up and I'm just sat there thinking about, you know, sleeping and I'm, you know, sleeping and I'm, and I feel horrific. Where would you start? So I would start, I think, by trying to understand what it is that is causing their sleep issues

because a lot of the individuals that I see who have been referred in with that sort of picture assume that they've got insomnia and they may not always have insomnia. So I think a key issue is that we are really, really poor witnesses to our own sleep. And what I mean by that is that we often, our experience of sleep is very different to the reality of sleep. When we bring in people into the sleep laboratory, it's not at all unusual.

And I will almost always ask this question when I'm going through a sleep study with one of my patients, is how much sleep do you think you got over the course of that night? And it's not at all unusual for people to say to me, well, I think I've got two or three hours sleep. occasionally even they say well I didn't sleep at all and then you look at their brain waves you look at the best objective marker that you've got of their sleep and you see that they've slept seven and a half eight hours so obviously what people are experiencing is really important because ultimately from my perspective I want to improve people's experience of their sleep and what it is that

they're they're complaining of but it's important to understand that what they're telling you may not necessarily be the objective truth. Now, that's really important when it comes to insomnia because it's not unusual for me to see individuals who, you know, they give you a story of very clear insomnia, but actually when you look at their sleep objectively, you find that although they say they haven't slept at all, they've slept seven hours,

but that seven hours has been completely disrupted by conditions like periodic limb movement disorder, which is these leg kicks associated with the restless leg syndrome, or sleep apnea, for example. Now, sometimes it's very evident from what people tell you that actually that's not the case that they've just got very clear insomnia. So that's really the starting point

to try and decide whether or not you feel confident enough in your clinical evaluation of them that you know what the issue is without doing a sleep study. And if you think that they do need a sleep study, then that's the point at which we arrange for that. It's also trying to understand some of the factors that might be driving their sleep difficulties. So, for example, were their sleep difficulties triggered by a life event?

Did they have sleep reactivity before this insomnia started? So were they one of those individuals who could sleep anywhere at any time, whatever they wanted, whenever they wanted to put their head down, or whether they kind of individual who the night before an exam, before a job interview, before a presentation, would lose sleep? Because that often is a very strong marker for developing insomnia. later on in life. And then it's also about trying to understand how the rest of their health is impacted by their sleep, but also how the rest of their health impact on sleep. So it's not at all

unusual for me to see individuals who have been started on medications for other reasons that have generated sleep issues, for example. You talked about this sort of obsession with sleep. And I was wondering in the case of the patient you've just described, would you encourage them to wear a sleep tracker? So first of all, I have to say that I'm not ideologically opposed to sleep trackers in general. I think that they are really, really good, for example, in research. You know, fantastic for research. It allows us to track sleep in very, very large numbers of individuals and try and work out how that correlates with whatever we're interested in. One of the major issues with sleep trackers is that the people who often use sleep trackers

are individuals who already are concerned about their sleep. So if you know that you're sleeping relatively little and you wake up feeling tired, then you probably know you're not sleeping enough. You don't necessarily need a sleep tracker to tell you that. If you're one of these individuals who has insomnia, who is spending plenty of time in bed but simply cannot get the amount of sleep that they need, then what a sleep tracker will do is it will increase your concern, your anxiety around your sleep. It's a very different picture from, for example, using a step tracker.

If you're sitting on the sofa and you look at your step tracker and you realize you've only done whatever it is, 5,000 steps, it's very easy to get up and go for a walk and do another 5,000 steps. If your sleep tracker is telling you you slept really badly, and you know you slept really badly and you're already worried about how badly you sleep, there's nothing that you can do on the basis of the information that your sleep tracker is giving you to suddenly go and get a little bit more sleep. sleep. And it's complicated by the fact that, you know, sleep trackers are pretty good at telling you how much time you spent in bed. They're reasonably good at telling you how quickly you dropped off to sleep. The reliability, the accuracy of these devices, most of these devices, drops off significantly when it comes to, for example, defining nighttime awakenings, defining stages of sleep, those kinds of things. So then you have that additional issue in the mix, which is that sometimes your sleep tracker may be you information that is not factually correct, and that may increase your anxiety further.

So I'm really very, very keen for people who have issues with their sleep, rather than just burning the candle at both ends, to put away their sleep tracker and actually go and have a chat with their GP or somebody who knows a little bit about sleep rather than relying on this sleep tracking technology. Do you think sleep trackers have had a negative or positive impact on sleep culture? I think that for those individuals who can fix their sleep in a very straightforward way by spending more time in bed. So the kinds of people I talked about that 20 years ago would be saying, well, you know, I only sleep five hours and, you know, because I'm busy doing X, Y and Z and I can get away with it, I think it's probably encouraged them to spend a bit more time in bed because they know they have a very clear, very clear bit of information that's telling them that. they're not sleeping enough.

But for the people that I see, the people who are already concerned about their sleep and who have difficulties with their sleep, I think it's been a very negative impact. And I have some reservations about, well, people like myself, sitting on these kinds of podcasts or writing in newspapers telling you, well, you know, if you don't get enough sleep,

you're going to die early, you're going to have all these negative health. consequences because for a subgroup of individuals who are already very concerned about their sleep that actually can cause problems. And I have seen individuals who, for example, have read books on sleep and how importance it is on sleep, who have ended up going into a spiral of insomnia and very catastrophic depression and anxiety as a result. So it's very, you know, it's very important to be clear that all of this is a double-edged sword. It's interesting with sleep trackers. I can see, I think it's worth me saying, that I am both an investor in WIP and I'm also sponsored by

whoop, but I also agree with the things you've said. So I've seen this sort of variance in how a sleep tracker can improve some people's lives and it can make other people more anxious in a way that's not helpful. So for me, my sort of testimony on it is I was one of those people you described earlier that thought sleep was take it or leave it. And when I started seeing a sleep track, it's kind of like that when I saw my brain for the first time, I did a brain scan, and I didn't even kind of like realize it was there and that I could influence it and that things I was doing without really thinking much were having this big impact. And for me, what it did is it allowed me to finally make this link between how much sleep I've had and then how I behave. Now, I thought

my behaviour was random before. But seeing that, when my sleep scores, were down, I was way more emotional, I was way more likely to eat crap. And the other thing that I saw, which was really interesting, was that when I had a glass of wine or two glasses of wine, three glasses of wine, that it just destroyed my sleep. And I never knew that before. And I was in search of reasons to quit alcohol anyway. And when I saw that, I quit alcohol forever. So I've not drunk since. So, and then for me, I have to also say, there are moments in my life where life happens and I know I'm not going to sleep and I don't pay attention to my sleep tracker. But there are other moments where I have a bit more control and that's when I kind of tune in to my sleep tracker.

I've also had parents message me a lot and say, listen, I've got a one year old, a two year old, a three year old, whatever. There's no point me wearing a sleep tracker because listen, I'm not going to get any sleep. And I also completely agree with them that there's really no point in that situation. I think there's a point when there's something you can do about it. Yeah. And that's kind of what you're saying. That's absolutely key. Yeah. So the point of doing anything like that is if there are very clear things that you can do on your own to close that loop, there's no point having information without being able to act upon it. And I guess if you are one of those individuals like yourself who very clearly can correlate certain things that they're

doing in their daytime lives with their sleep and how they feel subsequently, then great. You know, I guess I have a little bit of bias in that the people that I see are already struggling with their sleep already. And so it goes back, I guess we're completely in agreement. Yeah, we are, yeah. And it's so interesting because that's been a developing idea, because obviously my bias is always like, wouldn't you rather know? Because that's, you know, but then from doing this podcast, I've seen the comments and I've seen the struggle in from speaking to parents that struggling with their sleep and it's kind of sometimes just makes them feel worse about it. I think nuance is necessary on this issue.

What you just listened to was a most replayed moment from a previous episode. If you want to listen to that full episode. I've linked it down below. Check the description. Thank you. A couple of weeks ago I was travelling through Ireland with my team and I was telling them how I don't love when things in my life sit idle, whether it's my time, my energy, my health or my investments. If something has value, it should be working, even if just quietly in the background. And one of the most overlooked examples of this was when you're traveling and you're away from your home. Because when you're not in your home, they just sit empty and they're not doing anything for you, which is easy not to think about, but it's still a choice that you're making. We're choosing

not to get anything back from something that has real value, because our home can easily play a part in someone else's holiday experience. Airbnb is one of my partners, as you know, and hosting with them is a quick, easy way of changing that. You make your home available for dates that suit you, and instead of it just sitting there, someone else gets to experience and enjoy your home and your neighborhood and your city. Hosting on Airbnb, it also lets you make a little bit of extra money on the side, which you can put towards your next holiday. Your home might be worth more than you think,

and you can find out how much it's worth at Airbnb.ca slash host.