Sleep is a state of physical rest and reduced awareness. It is generally assumed that the main function of sleep is to allow the body and the brain to recover.
Cycles
Sleep consists of various cycles, with various forms of sleep; from light to deep. This theory is supported by observations that sleep is a period of physical recharging in which the production of the growth hormone shows a peak, the rate of cell division is at a maximum, and the creation of protein exceeds its breakdown.
Much more is now known about the mental and physical processes during sleep, thanks to the ‘electroencephalogram’ (EEG). This can be used to measure electrical brain activity. An EEG shows that sleep consists of various cycles, with various types of sleep.
To put it simply; the night can be broken down into sleep cycles. One cycle consists of five stages. These stages are distinguished by the degree of brain activity and eye movement.
During the first four stages, eye movement is slow. The stages therefore are called the ‘Non-Rapid Eye Movement’ (non REM-sleep). Only in the last stage do the eyes move rapidly, therefore this is referred to as Rapid Eye Movement or REM sleep.
Shallow sleep
Stages 1 and 2 are also called the shallow sleep. Stage 1 is the transition phase between being awake and being asleep. Eye movements become slow; you have problems keeping your eyes open and finally you fall asleep. Brain activity slowly reduces. After on average seven and a half minutes the EEG shows ‘sleep spindles’; repeating clusters of electrical activity. Stage 2 of non-REM sleep has started.
Stage 2 is the start of real sleep, but the sleep is still shallow. You will no longer wake up at every sound, but if you are woken up in this phase you don't have the feeling of having slept deeply. This stage takes 10 to 25 minutes.
Deep sleep
Stages 3 and 4 form deep sleep. Stage 3 starts approximately 30 minutes after falling asleep. This is the transition phase to deep sleep. Your breathing becomes regular, your heart rate drops, your muscles become completely relaxed.
After approximately 45 minutes, stage 4 starts. In this stage, breathing rate and heart rate are at their lowest. If you are woken from this stage of sleep, you will be disoriented and need time to realise where you are.
REM sleep
The final stage, stage 5, is the REM sleep stage. Rapid eye movements are characteristic of this stage of the sleep cycle. Stage 5 is also referred to as being the dream stage. If someone is woken from this stage, there is a high probability that he or she will be able to give a reasonably coherent report of the dream that they were having when interrupted. Other characteristics of REM sleep are a dramatic reduction in muscle tension, so that simple muscle spasms only occur now and again, and men sometimes get erections.
Following the REM sleep stage, you normally wake briefly (normally without knowing it) and the entire sleep cycle begins again. On average, we wake around 7 to 10 times a night; we are normally not aware that we have done so. The proportion of REM sleep varies per sleep cycle. In the first cycle it is short, in the third and the fourth in particular it increases. Non-REM sleep occurs comparatively more during the first and second cycles.
Waking
A characteristic of actually waking is that the body temperature rises. The rise in temperature is achieved by the body itself; as if the biological clock turns on a heating system. In addition to the rise in temperature, the level of the sleep hormone melatonin drops and the level of cortisol rises. Cortisol is a substance that in some respects can be compared to adrenaline; it must bring the motor of your body up to speed. As the morning progresses, the level of cortisol drops once more.
