July 25, 2024


Only the best news

RUSSELL FOSTER: How your body clock governs your weight, health and sex life

What do the words ‘body clock’ mean to you? Something you associate with jet lag and feeling out of sorts after a long-haul flight? 

Or the ticking of a woman’s biological clock? Or the fact some people function better first thing in the morning — and others later at night?

In fact, our body clock controls all of these very different things and much more, as I have discovered over the past 40 years working in this fast-emerging field.

Our body clock sets the very rhythms of our lives, affecting everything from how clearly we think and when our digestive systems are ready for food, to when our muscles are at their strongest and whether we will develop cancer. 

It affects our sexual compatibility, when we feel symptoms of illness, and the most effective time to take medication. It affects our decision-making skills and even how we communicate.

What do the words ¿body clock¿ mean to you? Something you associate with jet lag and feeling out of sorts after a long-haul flight? Or the ticking of a woman¿s biological clock?

What do the words ‘body clock’ mean to you? Something you associate with jet lag and feeling out of sorts after a long-haul flight? Or the ticking of a woman’s biological clock?

The clock controls our body’s daily (or ‘circadian’) rhythm in a 24-hour cycle. In so many cases, our ability to succeed or fail — including, for instance, dieting to lose weight — depends on whether we are working with or against these 24-hour cycles.

In this unique series for the Daily Mail and The Mail on Sunday, I will explain how to harness these circadian rhythms to revolutionise your health. Today, the focus is on diet, exercise and immunity. 

Tomorrow, I will reveal the life-changing new understanding about why symptoms of illness are worse at different times of the day and night — and when you should take your medication.

Make sure you read Monday’s Mail to see how your body clock could be affecting your relationship and sex life.

Four steps to reset your clock every day

This series is packed with practical advice for harnessing your body clock to boost health and happiness. But you can start straight away with these simple steps:

Get 30 minutes of light at dawn and dusk 

To keep our body clock in sync, we need to ‘reset’ it every day — the best way to do this is to get natural light 30 to 60 minutes after sunrise and before sunset (later than dawn is still OK, see main story).

Don’t fret about the blue light from screens 

There have been a lot of warnings about not using screens before bed — the idea is that the artificial blue light they emit tells your body the wrong time, and shifts your clock later.

In fact, the light from something like a Kindle or iPad is much, much weaker than dawn light, and one key study showed that four hours of use immediately before bedtime on five consecutive nights delayed sleep onset by only a few minutes, which is essentially meaningless. 

The real problem with late-night use of technology is that it increases mental alertness. This is what actually delays sleep.

Don’t go to sleep on an argument 

Talking to your partner in bed about problems might seem like a great way to catch up, but any disagreement increases levels of the stress hormones cortisol and adrenaline. 

This will delay and shorten sleep, and alter your body clock rhythms. So leave difficult conversations for the daytime and save bed for intimacy (sex lowers cortisol and makes you relaxed and sleepy).

Stick to regular sleep routine and no lie-ins 

Keep to a regular schedule on weekdays and at weekends. By catching up on sleep at weekends you’re missing the morning light, which means you’re not resetting your body clock.

This means that by Monday your body clock will be slightly out of kilter, making life more difficult in the long run.

The real problem with late-night use of technology is that it increases mental alertness. This is what actually delays sleep

The real problem with late-night use of technology is that it increases mental alertness. This is what actually delays sleep


The most obvious example of our body’s daily 24-hour rhythms is our sleep/wake cycle. This is tightly regulated by the body clock — which means that if we’re not asleep or awake when our body expects it, it can have major consequences. Sleep is essential for health and yet too often the importance of sleep is disregarded.

The trouble is, most of us assume we can do what we want, whenever we want. This assumption underpins modern 24/7 society, dependent upon night-shift workers to stock our supermarkets, run our global financial services, protect us from crime and, of course, care for the sick.

But night-shift work is not the only disruptor of our sleep and, consequently, of our body clock: many of us who work ‘normal’ hours curtail sleep as we try to squeeze more activity into a schedule that is already bursting at the seams.

We are, of course, not able to do what we want whenever we choose. For our bodies to function well we need the correct materials in the right place, in the right amount, at the right time of day.

Thousands of genes must be switched on and off in a specific order. Proteins, enzymes, fats, carbohydrates, hormones and other compounds have to be absorbed, broken down, metabolised and produced at a precise time for growth, reproduction, metabolism, movement, memory formation, defence and tissue repair.

All this requires our bodies to be prepared and ready at the right time — without precise regulation by an internal clock, our entire system would be in chaos.

So what is the body clock?

Our circadian rhythms are run by a master clock located within our suprachiasmatic nuclei (SCN) in the brain. They tick away independently, generating a rhythm of around about 24 hours. However, it is only around about 24 hours: they tick a little faster or slower, depending on the individual.

In this way, circadian rhythms resemble the mechanism of a grandfather clock which needs a daily adjustment. Without this resetting, the master clock in the brain will drift out of alignment with the 24-hour day.

And it’s more complicated than just one body clock. The SCN is our ‘master clock’, but we also have clocks within the cells of the liver, muscles, pancreas, fat, and probably every cell of the body — and all these other body clocks keep time with the master clock.

Think of the circadian system like an orchestra: when all the musicians are playing in sync, you get a symphony. But if they’re playing off-beat, which is what happens if the master clock isn’t set to the same time as all the cellular clocks, you get a biological cacophony, and the whole system begins to break down.

Your SCN body clock needs to be reset on a daily basis and key to this is light, especially around sunrise and sunset — we call this process ‘entrainment’.

Take a shower at night to support your immune system

The past two years spent battling a pandemic have brought into sharp relief the importance of our health and the vital role of our immune systems.

Science has brought us, for a while, effective vaccines for Covid-19. But there is still much to be learned about how we can protect ourselves from this virus and many others.

It’s early days, but there are at least two studies showing that sleep and circadian rhythm disruption in night-shift workers increase the chances of infection and hospitalisation with Covid.

This chimes with other studies showing that disruption to our body clocks impairs our immune systems — and that our individual responses to all sorts of infection change over the day. So, too, do our responses to vaccination and medication.

More at risk of colds at night

We now know that every aspect of the immune response is regulated by the circadian system.

Scientists have discovered that our immunity is generally turned up in the daytime, when we’re active and are more likely to encounter disease, while at night it is turned down, when the chances of encountering new pathogens is much reduced.

Our first line of defence is our skin, an incredibly effective barrier preventing disease-causing microbes such as viruses, bacteria and other pathogens entering the body.

However, at night — when, if working correctly, our body clocks have every system primed for sleep — our skin becomes more porous, and so more vulnerable to attack.

More water is lost from it and old, dead skin — the topmost layer of defence — is shed, with the highest rate of shedding occurring around midnight.

The loss of water means our skin is that bit drier, which is why anyone suffering with itchiness or conditions such as eczema and psoriasis will suffer more as the evening wears on. (More on this tomorrow in the Mail on Sunday.)

The increased permeability means we are more at risk of bacteria and viruses invading.

Meanwhile, our white blood cells, which attack invaders, are less driven to attack at night — guided by a circadian clock, they’re more active during the day, when we’re usually awake and logically more ‘at risk’ of encountering infection or injury.

And talking of injury, if you cut or burn your skin, studies suggest it will heal more than twice as fast if the injury was inflicted during the day than at night.

This all makes sense, as it’s much more likely that we will damage our skin while we’re awake and our bodies’ repair systems are primed for action during the daytime.

So how can you reduce these risks? The most obvious way is to work with our circadian rhythms and spend our nights asleep.

Of course, that’s not realistic for everyone — front-line workers in particular. So they should wear protective clothing at work, and all of us should shower at night and vigorously wash our hands and face before bed each night to remove pathogens.

Many of us shower in the morning, but good hygiene in the evening is potentially of much more value to our immunity!

Have a flu jab in the morning

Given that our defensive mechanisms rev up in the morning, it would seem logical to give vaccines then.

Indeed, early evidence certainly suggests this is a good idea.

In one study, elderly individuals were vaccinated against a strain of the flu virus either in the morning (9am to 11am) or in the afternoon (3pm to 5pm).

Those vaccinated in the morning had an antibody response three times higher than those vaccinated in the afternoon, when the immune system is beginning to wind down for the night.

And get a good night’s week 

Making sure you get a good night’s rest in the days following your jab could also boost its effectiveness.

One study found that people allowed to sleep for only four hours a night after the flu jab had less than half the level of protective antibodies a few days later than those who had slept well.

Similar results have been shown for hepatitis B and hepatitis A vaccination.


The eye detects dawn and dusk to entrain our SCN, which then sends out signals to keep all the other circadian clocks ticking at the right time, ultimately keeping the ‘internal day’ entrained to the 24-hour astronomical day.

That is why it is so important to expose ourselves to natural light in the mornings and dusk in the evenings, in order to keep our clocks ticking on time with each other.The most effective way to set your body clock is to experience natural light 30 to 60 minutes after sunrise and before sunset. (Light exposure later in the day, until noon, will also help — it’s just that sooner after sunrise is most effective).

I’m the last person to get up at dawn to herald the sun, but when I do wake, the first thing I do is open the curtains; I’ll also eat breakfast by the window. It doesn’t matter if it’s overcast because the light-sensitive cells in the eye ‘add up’ the light they receive (it just might take longer than on a bright day to get the message).

Interestingly, small inherited changes in our clock genes have been linked to whether we are a ‘morning’, ‘evening’ or ‘intermediate’ body-clock type — or ‘larks, ‘owls’ and ‘doves’. (Doves wake between 7am and 8am and want to go to sleep from 10pm to 12am).

Larks make up ten per cent of the population; owls, or evening types, account for 25 per cent of us, and the rest, in the middle, are doves. Larks like to sleep early and get up early, and it seems they have faster body clocks. By contrast, owls have slower clocks.

Together with light exposure, a regular pattern of sleep and wake (i.e. not getting up or going to bed later than your norm) is instrumental in keeping all these clocks ticking in time (eating at regular times is also important, as I explain on the next page).

Disturbing these patterns can lead to a phenomenon we call sleep and circadian rhythm disruption or SCRD — and this condition sets us up to be vulnerable for all manner of ill health.

Whether you’re a titan of industry putting in early mornings and late nights on a deal, or the parents of a baby dealing with feeding all night, the lack of sleep you experience puts your body under threat.

Part of the problem is that poor sleep can put your body’s response to stress (the ‘fight or flight’ response) into overdrive — pumping adrenaline and cortisol, as well as energy-giving glucose, into your system. These hormones raise your heart rate and blood pressure and prepare you to deal with an aggressor (little does your body know the aggressor is an insurmountable pile of paperwork, not a marauding gang or wolf attack).

Over time, it’s like keeping an engine in first gear. First gear gives you that wonderful acceleration. But if you keep your engine in first gear, you’re going to damage it.

The same is true of your body. If you’re constantly operating with an elevated heart rate, heightened blood pressure, and flooding the circulation with glucose, your body cannot run on this sort of ‘emergency’ setting for ever — you are effectively giving yourself long-term stress poisoning.

As a result, some of the most challenging diseases of our time are associated with SCRD. For example, the raised cortisol levels caused by lack of sleep lead to blood sugar imbalances, obesity and type 2 diabetes; it suppresses the immune system and can even increase the risk of dementia.

Lack of sleep also contributes to dreadful mistakes — studies have shown that after four consecutive night shifts the risk of having an accident of any kind rises by 36 per cent.

In fact, sleep loss has been linked to multiple notable industrial accidents, including the Chernobyl nuclear power plant explosion, the Exxon Valdez oil tanker spill, the explosion of the Challenger space shuttle and the Bhopal chemical plant disaster. It’s not called the graveyard shift for nothing.

Yet rather than being recognised as loyal friends, biological rhythms and sleep are frequently portrayed as enemies. What we should be doing is protecting our sleep, not trying to cut it short because ‘sleep is for wimps’.

But if sleep and light exposure are vital, they’re not the only factors that affect the daily resetting of our body clock: eating at erratic hours, drinking too much alcohol and vigorous exercise before bed (see next page) have an impact, too. This is because we’re doing things at times our bodies don’t expect it.

And we ignore our circadian rhythms at our peril. Because what we do when really does matter.

This is life-changing information that until now society has largely ignored. Circadian and sleep health is not taught in our schools or to our medical students. It is also absent in the working environment, in many cases.

It has left the economy in the hands of chronically tired and stressed individuals, and constitutes a major missed opportunity to improve health at every level.

Mahatma Gandhi said: ‘The future depends on what you do today’. And so, I want poor sleep and the widespread disruption of our circadian rhythms to become as anathema socially as smoking has become. I want burning the midnight oil to move — as puffing on a cigarette has — from a fashionable practice to an activity regarded by most as socially unacceptable and irresponsible.

With the right education, I hope our machismo culture of long hours and little sleep will go the way of the ashtray.

For the lives of both wise and foolish people all end in death, but those who are wise to their circadian rhythms will, on balance, live longer, be happier and lead more fulfilled lives. I hope that this series will help you become one of them.

When to work out

The more active you are, the more glucose — and stored glucose (i.e. fat) — you turn into energy. So is there an optimal time of day to exercise for maximum calorie burn?

I’m delighted to say that there is. In general, our muscle strength peaks between 4pm and 6pm, which coincides with our peak in core body temperature.

Increased body temperature will increase our metabolic (calorie-burning) rate — and increased muscle power will mean we can do more, and so burn more energy that way, too. Even when resting, we burn about 10 per cent more calories in the late afternoon and early evening, compared with the early morning.

And if you’re an evening type, exercising later in the day confers even more advantages!

A study involving athletes showed that larks, doves and owls (see previous page) all performed better as the day progressed, but owls did the best later in the day, with as much as a 26 per cent difference between their abilities at 7am compared with 10pm. That said, there are upsides to getting moving before breakfast. On an empty stomach, the body is still using stored fat as fuel, so early exercise will burn more fat.

The bottom line is: you’re more likely to burn stored fat first thing, but exercising later on will allow more vigorous exercise — and so more calorie burn overall. It’s a win-win, with advantages whatever time you work out.

Or you could hedge your bets, like some of my colleagues, who do a daily morning exercise of about 20 minutes, followed by a 30 to 40-minute block of activity later on.

Learn to time your meals right to fight off the flab 

At any given time, about half the world’s population is trying to lose weight. For millions of us, dieting is an over- whelming preoccupation.

But guess what? This, too, is influenced by our circadian rhythms. And if we ignore it, we will be undermining any efforts we’re making to maintain a good diet and take regular exercise.

Your body clock influences every aspect of your metabolism, from hunger and digestion to when hormones and enzymes related to your metabolism are produced.

Under normal circumstances we eat during the day — that’s why our body is primed so that saliva production rises over the day and falls at night. And (as we now know) our stomach naturally empties faster in the morning than in the evening.

But if anything throws this cycle off — such as eating very late at night, when the body isn’t ‘ready’ for food — it won’t take long for chaos to ensue and our metabolism to become far less efficient.

This has been shown in mice studies, where the rodents were made to feed at a time they should normally be asleep. Remarkably, the clocks in the animal’s livers, muscles, gut and other organs ‘moved’ their circadian rhythms to coincide with the new feeding time.

However, the master clock in the brain remained locked on to the old light/dark cycle. As a result, the master and peripheral clocks were no longer aligned with each other — and the normal metabolic pathways that underpin the different demands of sleep and activity were severely disrupted.

Long term, this led to obesity and metabolic problems such as type 2 diabetes.

So what does this mean for us? Studies have shown a clear link between night-shift work, metabolic syndrome (a group of conditions that includes high blood pressure and obesity) and high blood glucose, or sugar, leading to type 2 diabetes. This is because the workers are forced to work and eat when their bodies expect them to be asleep.

The same could happen to anyone who regularly eats when they should really be asleep. The disruption means we are far less likely to mop up blood glucose because we can’t release sufficient insulin (which acts to lower blood glucose) at this time of night which, in turn, greatly increases our chances of weight gain and type 2 diabetes.

The disruption to our body clocks also sends our hunger hormones haywire. These are leptin, the ‘satiety signal’, and ghrelin, the ‘hunger hormone’ which stimulates appetite.

Recent studies have shown that leptin release peaks at about 2am (when you’re sleeping) and the low point is around noon (when you’re active). Normally, high levels of leptin at night suppress appetite so that hunger doesn’t disrupt your sleep.

Meanwhile, ghrelin is secreted by the stomach during the daytime and increases in anticipation of meal times.

The release of these hormones programmes our bodies to expect a large meal in the morning and at lunchtime, with a smaller meal in the evening and nothing at night.

But people whose sleep is interrupted have consistently lower levels of leptin and raised ghrelin, increasing their hunger so they eat more food. A classic study examined the impact on healthy young men of having only four hours of sleep on two consecutive nights.

In these young men, blood levels of leptin decreased by 18 per cent, while ghrelin (to remind you, that’s the hunger hormone) increased by 24 per cent. The same increase was recorded in their levels of hunger and appetite. These findings strongly suggest that we are effectively programmed to consume more calories when we are deprived of sleep.

Try to breakfast like a KING!

The good news is that eating at the right time can truly maximise our chances of maintaining a healthy weight.

The philosopher and physician Maimonides (1138–1204) is remembered for many things, but by me most pertinently for the saying, ‘Eat like a king in the morning, a prince at noon, and a peasant at dinner’.

This philosophy makes him the founding father of ‘chrononutrition’, the science that’s identified that when we eat is just as critical for health as what we eat.

While in Maimonides’ time the largest meal of the day typically was breakfast, over time the main meal of the day has — with the advent of artificial light, industrialisation and changes in working practices — moved later and later.

Today, long commutes, irregular hours, the pressures of school work and the availability of microwave food have all pushed the major meal of the day to an irregular mid to late-evening slot.

If you were designing a schedule to be particularly bad for our circadian-regulated metabolism, this would be it.

For as I’ve explained, eating in the evening when your body isn’t expecting it greatly increases your risk of developing glucose intolerance, type 2 diabetes, weight gain and obesity.

A detailed study in 2013 compared people on the same 20-week reduced-calorie diet, consuming most of their calories either early or late in the day. Those who ate late in the day lost less weight, more slowly.

Importantly, the same meal in the evening results in higher levels of blood sugar than if eaten in the morning — the result of the normal body clock cycle preparing itself for a sleep- time metabolism with no food intake.

The simple answer? Go back to the old-fashioned principle of a big breakfast, a hearty lunch and a light, early, evening meal. And don’t raid the fridge at midnight!

n Adapted from Life Time: The New Science Of The Body Clock And How It Can Revolutionise Your Sleep And Health by Russell Foster, published by Penguin Life on May 19 at £16.99. © Russell Foster 2022. To order a copy for £15.29 (offer valid until May 21, 2022; UK P&P free on orders over £20), visit mailshop.co.uk/books or call 020 3176 2937.