What is Sleep?

See also: How to Sleep: The Importance of Sleep

The exact reasons why we need sleep remain a mystery.  However, it is known that in order for humans (and many other species) to be able to function optimally, periods of sleep are required.

During periods of sleep our bodies repair muscle and our mind stores, organises and links memories.  At the same time, important hormones that control essential bodily functions are regulated in the bloodstream, our immune systems are strengthened, and the risk of serious diseases is reduced.

This page covers the mechanics of sleep. Also see our page: How to Sleep - The Importance of Sleep, which includes information on promoting good sleep, the dangers associated with lack of sleep and factors or triggers that can make us sleepy or lethargic.

You may also want to assess your levels of daytime sleepiness with the Epworth Sleepiness Scale.

How Much Sleep Do We Need?

There is no hard and fast rule as to how much sleep we each need.

Different people need different amounts of sleep and at different times in their lives – your optimal sleep time may vary from that of other people.

Children and teenagers generally need more sleep than adults.

Most adults require between 7 and 9 hours of sleep a night (in a 24-hour period). Older people need about the same amount of sleep but may find it more difficult to sleep.

There is an association between illness and too much/too little sleep

Scientists have found a link between the likelihood of developing an illness, condition or disorder, and the amount of sleep that we get.

On average, people who get significantly more or significantly less than 8 hours’ sleep per night are more likely to develop a condition or disorder.

However, this is not to say that the lack of or excess sleep causes that condition (and for more about this, see our page on Correlation). Instead, it is more likely that the two are associated. For example:

  • You may have noticed that when you are coming down with a cold, you need more sleep. Needing more sleep may therefore be a sign that you are developing a health condition.
  • People who are less fit may exercise less, which can lead to problems with sleeping (for example, inability to fall asleep quickly). It can also affect health more generally.

This suggests that if you find that you routinely need significantly more or are getting significantly less than 8 hours’ sleep per night, it would be worth talking to your doctor.

On average we spend about a third of our lives sleeping, but this should not be considered wasted time. Sleep is essential for our performance and wellbeing during waking hours.

Some people have famously claimed to function well on less sleep: Margaret Thatcher, Bill Clinton and Madonna have all reported that they need only 4 hours of sleep a night,  but some experts believe that such people are simply unaware that they are in fact more tired during waking hours than the norm.

Quality Sleep and the ‘Sleep Cycle’

Quality sleep is defined as uninterrupted sleep over a period of approximately seven to nine hours.

If sleep is interrupted then there is less chance that the body will have had time to perform all of its essential tasks.

During the night we alternate between different types of sleep, and this is known as the sleep cycle or sleep architecture.

There are two main types of sleep which make up the sleep cycle: non-rapid eye movement (NREM) and rapid eye-movement (REM) sleep.  Approximately 75% of sleep time is made up of NREM sleep, with the remaining 25% being REM sleep.

NREM Sleep

NREM is typically categorised into three distinct phases:

  • NREM Stage One

    This is the transition stage of being awake and beginning to fall asleep.  Often people do not think of this stage as ‘sleep’ and feel more awake than at other stages of sleep.  It is not uncommon to still be broadly aware of surroundings during this stage of sleep, and the body may experience involuntary jerking/twitching movements known as hypnic jerk.  Hypnic jerks are normal but more prevalent in younger people, but stress and other factors may increase the frequency of hypnic jerks.

  • NREM Stage Two

    By the time we reach stage two of NREM sleep our bodies start to become more relaxed, breathing and heart rate slow and we become less aware of environmental factors such as sound and light.  Temperature becomes a more important factor during stage two of NREM sleep as our body temperature naturally drops; if our surroundings are too hot then we can feel uncomfortable.

  • NREM Stage Three

    This is the deepest phase of NREM sleep and the most beneficial to overall wellbeing as, during this phase the body works on rejuvenation.  Stage three of NREM sleep is characterised by slower breathing and heartbeat leading to a lower blood pressure.  The heart provides more blood to relaxed muscles which promotes repair and growth.  A series of hormones are released into the bloodstream which helps to regulate and control tissue and muscle growth and repair as well as regulating our metabolism and appetite.

REM Sleep

REM sleep occurs for approximately a quarter of the time we spend asleep. This type of sleep is characterised by fast darting eye movements and is the time when we are most likely to experience dreams.

REM sleep does not occur until approximately 90 minutes to two hours after we first fall asleep, it lasts for relatively short periods of time but repeats during the night at approximately 90 minute intervals. The duration of REM sleep sessions increase during the night: longer periods of REM sleep occur the longer uninterrupted sleep is maintained.

NREM sleep is thought to be more important to the body whereas REM sleep is more important to the mind (brain), although the two types cannot be separated. 

During REM sleep the brain is more active than during NREM sleep, hence dreaming, and our bodies become very relaxed as blood is concentrated in the brain rather than the muscles. During REM sleep the body is able to restore mental and physical energy most efficiently.


Lots of changes occur in the body and mind when we awake from sleep.

These changes will vary with the quality and duration of the sleep received. Waking from early stages of sleep is different from waking after a full uninterrupted quality sleep when the body has had time to complete a successful sleep cycle.

When we wake our heart rate, blood pressure and breathing rate all rise as does our body temperature.  Various hormone levels change, notably cortisol (the stress hormone) rises to enhance the feeling of alertness in the morning.  Our major internal organs switch to ‘wake mode’ and begin to function optimally.

The waking process can take a little time before we feel refreshed and ready to go, and it is normal to feel groggy for a little while after waking from a deep sleep.

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Having covered the mechanics of sleep continue to our page: The Importance of Sleep, which includes how to maximise your chances of getting a good night’s sleep (sleep hygiene), the dangers of not getting enough sleep, triggers for sleep and things that can make us drowsy outside of normal sleeping times.