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Professor Russell Foster reveals how resetting your body clock can revolutionise your slumber

When people find out that my work involves sleep, often the first thing they want to know is: What is the secret to getting enough of it?

The answer can be complicated, but one of the most important factors, counterintuitive though it might sound, is getting the right amount of light at different times of the day.

But that is being undermined by modern life. Most of us assume that we humans are above the biological rules that dictate the activities of the rest of life on Earth — and that we can do what we want, at whatsoever time we choose.

This assumption underpins our 24/7 society, and an economy that’s dependent upon people working at night to stock our supermarkets, clean our offices, run our global financial services, repair our rail and road infrastructure, maintain the law and, of course, care for the sick and injured.

The answer to getting enough sleep can be complicated as we try to squeeze in leisure activities after working hours (Stock image)

Although working at night is the most obvious disruptor of sleep, many of us are also sleeping less as we try to squeeze more and more work and leisure activities into daily schedules already bursting at the seams. So we push these activities into the night.

Our full-scale occupation of the night has been possible as a result of the widespread commercialisation of electric light since the Fifties. This extraordinary and wonderful resource has allowed us to declare war upon the night — and, without really appreciating what we have done, our sleep has been the luckless victim.

Yet, as I explained in Saturday’s Daily Mail, sleep disruption can lead to serious health problems.

We are not, of course, able to do what we want at whatever time we want. Like almost all life on this planet, we are slaves to the rhythm set by the Earth’s 24-hour rotation upon its axis.

All of us have an internal 24-hour body clock that advises us when is the best time to sleep, eat, think and undertake myriad other essential tasks. There are optimum timings for different body functions and processes.

For instance, from around 6am, your blood pressure starts to rise sharply from its low point. This is in anticipation of waking up and the need to get more oxygen and nutrients to the organs in readiness for activity.

Your production of the hormone cortisol also peaks first thing (between 6am and 8am) to mobilise the body for action by increasing heart rate and the release of glucose into the circulation for energy.


In addition to the colour and timing of light, the intensity of it is critical to the timing of our body clock (see main copy below).

And the problem is that many of us are not getting enough light to set our body clock. Compared with natural outside light, even in winter, artificial light in offices and factories is very dim.

Overall, natural light is some 50 to 250 times brighter than most artificial light we encounter.

Light intensity affects the timing of our body clock, writes Professor Foster

Light intensity affects the timing of our body clock, writes Professor Foster

Light is measured in lux. Natural light is around 10,000 to 100,000 lux, depending on the weather, compared with the 300 to 400 lux in the workplace (or 800 to 900 lux you would encounter close to a 60 watt bulb at home).

That’s why, no matter how many years workers spend on permanent night shifts, nearly all find their clocks don’t adapt to the demands of working at night. After leaving work in the morning, the night-shift worker will move from the dim light in the workplace to bright light outside, and their clocks lock on to the brighter light signal, setting them to daytime.

People who don’t experience much natural light at all, such as nursing home residents or commuters (during the short days of winter), have a different problem: they don’t get enough light to set the clock properly and, just like the blind veteran Mark Threadgold (see previous page), their body clocks drift — and with them, the times when they sleep and wake.

Most of them will want to go to bed later and get up later.

There will also be times when they find it difficult to sleep at night, while wanting to sleep during the day.


To find out how bright (or dim) your workplace or home is compared with natural light, you can buy a lux meter for around £20 online. You could then even buy bright artificial lights to help set your body clock in the morning and see if this makes a difference to the times you sleep and wake.

One option is a light box, which simulates the dawn and dusk cycle (slowly brightening then fading) to help stabilise your body clock.

The main thing to look for is that they produce enough bright light, in the region of 2,000 lux or more. Most of us need to expose ourselves to this level of light for at least 30 minutes in the morning.


At the other end of the day, after 7pm, your body temperature starts to drop in preparation for sleep.

At the same time, growth hormone levels increase, promoting the building and repair of tissues. Even urine production slows down at night, so we don’t have to wake and go to the loo too often.

For our bodies to function properly, we need the correct materials — such as hormones and proteins — in the right places at the right time of day. Thousands of genes also have to be switched on and off in a specific order. Enzymes, fats, carbohydrates and other compounds have to be absorbed, broken down, metabolised and produced at a precise time for growth, reproduction, metabolism, movement and repair.

And to do all this, our body functions and processes must be prepared and ready at the right time, guided by the body clock.

Without precise regulation, our daily cycle of activity and sleep would be in chaos.

Sleep is when many essential activities occur, including memory formation and problem-solving by the brain. Disrupting our sleep pattern can lead to serious consequences for our health.

And this is where light — or, rather, the light and dark of day and night — comes in.

A wristwatch is of no use unless it’s ‘set’ to the local time, and this is equally true for our body clock, which is set by the daily pattern of light and darkness.

In this way, our internal biological clock and the external environmental day are aligned, and this allows us to do the right thing at the right time.

All this week in the Mail, I will be explaining more about these 24-hour cycles and I will give you my step-by-step ‘prescription’ for tackling tiredness.

Today, I focus on one of the simplest but most effective ways to improve your sleep: getting the light right.


The idea of an internal clock is not a new one. We’ve actually known about these kinds of clocks (or ‘circadian rhythms’) as far back as 1729 — not in humans, but in a plant called Mimosa pudica.

Known to many gardeners, this plant has delicate leaves that fold up at night and open during the day. Jean-Jacques d’Ortous de Mairan, a French astronomer, observed that Mimosa leaves continue to show this rhythmic folding and unfolding for several days in complete darkness.

Over the next few hundred years, such daily rhythms, persisting in constant darkness, were recorded in many plants and animals, but it was only about 60 years ago that researchers began to study these rhythms in people.

One key study involved university undergraduates being housed in an underground bunker in Bavaria, isolated from any external environmental time cues.

Their sleep and wake cycles, body temperature, urine production and other biological functions were measured over many days and were shown to have a rhythmic pattern of about 24 hours.

And we now know that what makes these body clocks tick is a master clock, consisting of a pair of structures deep within the brain, known as the suprachiasmatic nuclei.

There are clocks in the cells of probably every organ and tissue of the body, including the liver, muscles, pancreas and fat tissue. But the suprachiasmatic nuclei act rather like a conductor of an orchestra, providing a time signal that coordinates all the instruments.

An internal clock inside our brains helps control when we feel tired and go to sleep (stock)

An internal clock inside our brains helps control when we feel tired and go to sleep (stock)

Without the conductor, everything falls apart and, instead of a symphony, you have a biological cacophony — and a failure to do the right thing at the right time, not least our daily cycle of being asleep or awake.

One very striking feature of circadian rhythms is that they do not run at exactly 24 hours, and can tick a little faster or slower. In this way, they resemble an old mechanical grandfather clock that needs a slight daily adjustment to make sure the clock is set to the ‘real’ astronomical day.


Even just a few nights’ sleep loss can have a big impact on your emotional and brain function. You’re more likely to experience mood swings, irritability and anxiety — and it can also affect your metabolism.

A 2004 study found that getting only four hours’ sleep per night sent the glucose metabolism of healthy young men into meltdown. Put simply, their bodies were unable to regulate their blood glucose and appetite. Two gut hormones, ghrelin and leptin, seemed to play a key role.

Sleep loss can make you more likely to experience mood swings, irritability, anxiety - and it affects your metabolism

Sleep loss can make you more likely to experience mood swings, irritability, anxiety – and it affects your metabolism

Ghrelin is produced by the stomach and signals hunger, particularly for sugars; leptin is produced by fat cells and is a signal for fullness. Together, they regulate hunger and appetite.

Restricting the sleep time of the young men under laboratory conditions for seven days caused their leptin levels to fall by 17 per cent and their ghrelin levels to rise by 28 per cent, and increased their appetite, especially for fatty and sugary foods.

Such sleep disruption may explain why shift workers have a higher risk of weight gain and type 2 diabetes — and to a lesser extent why, when we’re tired, we seek out high-calorie foods.

People who lack sleep are also more likely to smoke and drink (probably related to stress).


For the suprachiasmatic nuclei master clock, this daily adjustment is performed by the cycle of light and darkness. Without this resetting, the internal day would soon drift and be out of sync with the environmental day/night cycle.

Many of us have experienced a severe form of this mismatch in the form of jet lag.

After travelling across multiple time zones, our body clock and the local time do not match, and we want to sleep and eat at the wrong times. But we do eventually catch up with the new time zone. The question is: How?

I cannot emphasise enough that a clock is of no use unless it is set to local time — and for most plants and animals, the signal that aligns the internal day to the external world is light, and especially light at sunrise and sunset.

This is evident from the experiences of people who have lost their sight, who drift through time, with periods when they get up and go to bed at the correct hours, and other times when they want to sleep, eat and be active at the wrong time of day.

Mark Threadgold, 52, a former soldier supported by the charity Blind Veterans UK, is a typical case. Twenty years ago, he was left blind by an accident at work.

While he has adapted to his sight loss, he’s struggled with sleep, because without the cues of light and darkness, his body clock drifts. He starts by falling asleep around 10pm and waking around 5am, but the time he drops off then shifts by an hour every day or so.

At some point in the month, he’s unable to sleep at all in the night. A few weeks later, he’s asleep at 10pm again. (I’m working with Blind Veterans UK to help people like Mark — and am hopeful that we will soon have a drug that will be able to ‘set’ the body clock, fooling it into believing it has seen light, and restoring a sense of time.)

As to how the body clock works, my team and I recently made an exciting discovery.

The light signals that the suprachiasmatic nuclei master clock rely upon are received via light-sensitive cells (called photoreceptors) in the eye. There are two types of photoreceptor, rods and cones, used for vision.

For decades it was thought that these also detected sunrise and sunset to regulate the body clock. But we have discovered a third group of photoreceptors, called photosensitive retinal ganglion cells, and these are critical for setting the clock.


These photosensitive retinal ganglion cells are most sensitive to blue light (the type that is emitted in daylight and from many electrical devices) and seem to be ‘tuned’ to detect the blue light of dawn and dusk.

When we think of sunrise and sunset, our image is of a red rising or setting sun — but in fact, above the sun it is blue.

Indeed, there’s actually more blue light at sunrise and sunset than at any other time of the day.

This does not mean that the photosensitive retinal ganglion cells detect only blue light, but it takes more non-blue light to get the same effect on the body clock — so exposure to natural light in the hours after sunrise and before sunset will adjust the clock, but much less effectively.

Exposure to natural light in the hours before and after sunset will adjust the body clock (stock)

Exposure to natural light in the hours before and after sunset will adjust the body clock (stock)

So, what does this all mean?

Simply, that the most effective way to set your body clock is to experience natural light 30 to 60 minutes after sunrise and before sunset.

Easier said than done, you might think, especially in the sub-zero mornings of winter. But while this dawn light is the most effective, light exposure later in the day, until noon, will also help — it’s just that sooner after sunrise is better.

I’m certainly the last person to get up at dawn to herald the sun, but when I do wake up, the first thing I do is open the curtains; I’ll also have my breakfast and morning coffee sitting by the window at home or at work.

Other tricks you could try include getting off the bus, and especially the Tube, a stop early to walk in the natural daylight.

It doesn’t matter if it’s overcast because the photosensitive retinal ganglion cells ‘add up’ the light they receive; it might take longer than on a bright day to get the message, but the light message is received eventually.


As the photosensitive retinal ganglion cells are most sensitive to blue light, manufacturers have produced lights enriched with blue light that can be used to artificially set your body clock when you can’t experience the natural dawn and dusk cycle.

Similarly, much has been made of reducing ‘blue light’ from devices such as mobile phones and laptops (using red filters on screens, for instance) to avoid delaying the body clock and causing sleep problems.

This may sound like heresy but, as I will explain in greater detail later this week, it’s not the light emitted from these devices that’s the problem, as there’s not enough of it to have real effect.

The real issue is staying up half the night using the device — and getting less sleep.


It’s not only the light that matters: your body clock timing is also partly down to your parents. Have you ever asked yourself why is it that you like to go to bed early and your spouse does not?

Thanks to research by three U.S. scientists who shared the Nobel Prize in 2017, we now know how our ‘clock’ genes and the proteins they produce generate the body clock.

Slight differences in these clock genes have been linked to whether we are a ‘lark’ (a morning person) or an ‘owl’ (an evening person).

Genes can also control the body clock, deciding whether you are a lark or an owl

Genes can also control the body clock, deciding whether you are a lark or an owl

Most of the population are actually neutral types, which means they broadly conform with the social norm in terms of sleep and wake times.

Morning types prefer to go to sleep early and get up earlier than the social norm, and it seems that they have faster body clocks.

By contrast, evening types, or ‘owls’, have slower body clocks and prefer to go to bed later and sleep in for longer compared with the societal norm.

So, by their contribution to our genes, our parents are still telling us when to get up and when to go to bed!

Our body clock type (or chronotype) may be inherited, but it’s affected by a number of factors, including age and light. And you can cheat the system. For instance, if you’re an owl but need to get to work early, make sure you get plenty of either natural or artificial light in the morning (and avoid light at dusk), which will advance the clock, making you get up earlier and go to bed earlier.

If you’re a lark and want to be more like the rest of society, get evening light (and avoid morning light), which will delay the clock, making you go to bed later and get up later.



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