What is sleep, and why do we need it?

Sleep seems to be as important to our health as diet and exercise - without it we cannot function properly and may even become ill.
 
We do know when we sleep, our bodies rest - conserving energy and decreasing blood pressure, heart rate, breathing and body temperature, but our brains remain active - laying down memory, restoring daytime mental function and carrying out tasks that lead to physical growth.

Lack of good sleep can lead to excessive daytime sleepiness, tiredness and lethargy, morning headache, poor memory, anxiety and depression. People who regularly don't sleep well are also more likely to have accidents, abuse substances such as alcohol, and suffer greater illness and disease.

The type and duration of sleep we have changes during our lifetime.  

Benefits of sleep are not just mental, scientists believe sleep can:

Different phases of sleep

Rather than being one uniform state, sleep has 2 distinct phases: rapid eye movement (REM) sleep and non rapid eye movement (NREM) sleep. NREM sleep can be broken down into 4 gradually merging stages. When scientists research sleep, they often look at the brain’s electrical activity, measured with an electroencephalogram (EEG). The different stages of sleep have different features and look different on an EEG, as shown in the table below.

The different phaes of sleep
Sleep Stage		Features	EEG Trace
Stage 1 NREM	Light sleep	Transition between sleep and waking. Person is easily roused. Fleeting thoughts. Eyes move slowly. Reduced muscle activity.
Stage 2 NREM		Eye movements stop Brief dreams
Stage 3 NREM	Deep Sleep	Body temperature falls Person is hard to awaken
Stage 4 NREM		Body temperature falls further Brain’s use of energy decreases Muscle tone decreases slightly Person may sleepwalk
REM		Eyes move rapidly to and fro Most dreams occur Heart rate and blood pressure increase Limb muscles are paralysed Brain is very active and uses a lot of energy.

REM sleep happens about every 90 minutes during the night, and REM periods lengthen as the night progresses while the time spent in NREM stages 3 and 4 decreases. Over a typical night, people spend about 20 per cent of the time in REM sleep and 80 per cent of the time in NREM sleep. However, these proportions vary throughout life; babies spend at least half of the time they’re asleep in REM sleep, but this proportion decreases as children mature; from late childhood onwards only 20 per cent of the night is spent in REM sleep.

Make regular deposits to your sleep bank!

Getting enough sleep can be likened to banking your savings – if you take sleep out of the account, you have to put it back to restore the balance, there’s no other way to catch up. Lack of sleep night after night leads to sleep debt, where your performance and sleepiness both get progressively worse.

When this happens your attention, learning and physical performance all suffer. REM sleep in particular has been linked to learning and to creating new memories. To restore your sleep balance, you need at least two nights in a row of unrestricted good quality sleep.