The science of sleep

Published by Bupa's Health Information Team, December 2010.

You spend about a third of your life asleep – but have you ever thought about why you sleep? What is it that makes you feel tired and how does your body know how much sleep you need? Do you sleep soundly at night or are you disturbed by nightmares or easily woken by your partner's snoring?

Your need for sleep is controlled by various biological processes in your body, which are also influenced by external factors such as sunlight. This is why jet lag and working night shifts can disrupt your sleep.

This article will help you to understand the science of sleep – so you'll be all clued up on how to get a good night's sleep.

Why do we need to sleep?

Despite extensive research over the last few decades, sleep remains a bit of a mystery. The reasons why we need to sleep are still not fully understood and have long been the subject of debate. There are several theories, but the two that are generally most accepted are that sleep may either allow your body to replenish energy stores and repair itself, or give you the chance to conserve energy. It’s also been proposed that sleep can aid learning and may give the body time to organise all the memories of the hundreds of events we experience every day.

We know much more about the effects of not getting enough sleep. Lack of sleep is associated with reduced concentration and feelings of tiredness, irritability and depression – which suggests that it is in some way important for preserving our mental function.

What makes us fall asleep?

You may never have given much thought to what causes you to fall asleep. Try to stay awake for too long and eventually you’ll feel so tired that you won’t be able to help dropping off. But how does your body know when you need more sleep – and why do people always tend to feel sleepy at night-time?

There are various chemical processes in your body that control sleep. Some of these keep track of how long you have been awake and when you need to sleep, and others determine what the best time of day is to sleep. These processes interact with each other and with environmental ‘cues’ to make you feel sleepy at about the same time every day.

The exact mechanisms are still unclear, but a number of chemicals that occur naturally in the body and work in the nervous system are thought to play a role in making you feel tired and telling your body when you need to sleep.

You also have an internal body clock, which is governed by daylight, making you feel awake in the daytime and sleepy as night approaches. As it gets dark, a special light-sensitive pigment in your eyes called melanopsin registers the lack of daylight and sends signals to your brain, setting off the various processes that make you feel tired.

What happens when you go to sleep?

So what actually happens when you nod off for the night?

There are two different types of sleep – rapid eye movement (REM) and non-rapid eye movement (non-REM) sleep, which includes four stages.

You go through the different phases of sleep in regular cycles during the night, moving through the stages of non-REM sleep, then into REM sleep and back again. Each full cycle takes about an hour to an hour and a half.

The sleep cycle

• Light sleep (non-REM stages one and two) – these are the first stages of sleep, beginning as you start to feel drowsy. As you fall asleep, your muscles relax, your heart rate slows and your body temperature falls. You can easily be roused from light sleep.
• Deep/slow-wave sleep (non-REM stages three and four) – your blood pressure falls in this stage and you sleep more heavily. You are hard to wake from this stage and will be confused if you’re woken up. It’s in this stage of sleep that some people can sleep talk and sleep walk.
• REM sleep – your brain is active in this type of sleep. Your eyes move rapidly from side to side but the rest of your body will be relaxed. This is the time when you dream. If your dreams are particularly vivid, you may experience nightmares.

The first two stages of non-REM sleep make up about half of your night’s sleep, with the ‘slow wave’ stages and REM sleep accounting for about a quarter each. However, how much of each type of sleep you get can change as you get older – babies have a much higher proportion of REM sleep, and the amount of slow wave sleep we get generally decreases as we age, with some older people not having any at all.

Restless night?

It’s normal to wake up during the night every couple of hours for a minute or so. You won’t usually be aware of these brief interruptions to your sleep. However, if you’re disturbed by something when you wake up – for example by noises such as snoring or if you’re feeling anxious about something – it may make it harder to go back to sleep.

If this becomes a regular problem, it can start to make you feel tired during the day. See our article on How to get a good night’s sleep for more information.

For sources and links to further information, see Resources.