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What is Sleep?

By: Wendy Jacob - Updated: 19 Mar 2013 | comments*Discuss
Age Circadian Rhythm Dreaming Sleep

Sleep is not only a period of rest during our waking lives, but is needed to keep our bodies and minds functioning smoothly. Like many things in life, we rarely consider its importance until there is a problem such as insomnia.

Sleep is complex and just like when we are awake, there are differences between individuals and changes during our lifetime. Neither is sleep inactive, the idea that sleep is a time for 'recharging' the mind and body (rather like a battery) is deceptive.

Sleep is a highly important function, essential for growth and development and for psychological processing. Although we may not have control during this time, it is not inactive. The quality and quantity of sleep we receive impacts on our health, relationships, work and society.

Watching others sleep helps us understand what is happening within the brain and the body. Even without laboratory equipment the different stages of sleep may be seen - movement of the eyes beneath the eyelids, changes of position, periods of stillness.

Studying Sleep

The creation of specialist 'sleep laboratories' has created a better understanding of what happens to the brain and body during sleep.

Scientists use a number of instruments and measurements, as well as observation to enable them to analyse the changes that take place.

The measurements are used to show the changes during the different stages of sleep and how they differ or compare with being awake. These changes include:

  • Changes in electrical activity in the brain.
  • Changes in muscle tone.
  • Eye movements.
Electoencephalography (EEG) measurements show electrical activity in the brain. Electrodes on the skin measure changes and identify different stages of sleep.

Electromyography (EMG) measures muscle tone, which changes during sleep and differs from being awake.

Electro-oculography (EOG) measures eye movements.

By analysing the results of these measurements, scientists are able to identify what is happening in the brain and body during the different stages.

Sleep Stages

Sleep can be divided into four stages and two distinct states - REM and non REM (deep sleep). The Rapid Eye Movement (REM) state is when the eyes can be seen to move under the eyelids and dreaming occurs. During deep sleep the body slows down, the muscles relax, breathing and heartbeat become slower and blood pressure decreases.

The four stages can, for an adult, be divided into 90-minute cycles. This starts with some light sleep which is roughly half of all sleeping time, then deep sleep, accounting for about 20 per cent, and finally the lighter REM sleep when dreams occur which is about 20 per cent.

Changes During Sleep

As we fall asleep and lose consciousness there is a steady change in EEG measurements as brain activity slows. The four stages of sleep are identified by changes in the brain and muscle tone. The discovery of REM sleep, where the eyes make rapid movements and the muscles are deeply relaxed, was so important that the other stages are now called non REM sleep.

During REM sleep activity in the brain is similar to being awake. This is the time when we have dreams and the body is relaxed and still. This deep muscle relaxation is believed to be nature's way of protecting us from 'acting out' our dreams.

Some analysts suggest that dreams, where the dreamer recalls being paralysed in the face of danger or being unable to scream, are related to this state. REM sleep is also called Pardoxical Sleep because of the activity in the brain and the relaxation in the muscles. Research has shown a relationship between the movements in the eye and the level of relaxation in the muscles.

REM sleep and non-REM sleep continues in ninety-minute cycles throughout the night. The amount of REM sleep lengthens as the night progresses. When REM sleep is lost, sleepers make up for this by spending longer in this stage on subsequent nights. One theory suggests that it is during REM sleep that the mind processes the experiences of the day.

Other research suggests that the amount of eye movements during REM sleep may relate to the amount of negative emotion in dreams.

Sleep Patterns

Why do we sleep at night and find it hard to adjust to changes in our routine, especially when travelling? Wouldn't it be useful if we could accumulate some extra sleep one night and then stay