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4.1 What Is Sleep?
Man, like all living creatures has to have sleep. Despite a great deal of research, the purpose of sleep is not fully understood. Sleep is a natural state of reduced consciousness involving changes in body and brain physiology which is necessary to man to restore and replenish the body and brain.
Sleep can be resisted for a short time, but various parts of the brain ensure that sooner or later, sleep occurs. When it does, it is characterised by five stages of sleep:
Stage 1: This is a transitional phase between waking and sleeping. The heart rate slows and muscles relax. It is easy to wake someone up.
Stage 2: This is a deeper level of sleep, but it is still fairly easy to wake someone.
Stage 3: Sleep is even deeper and the sleeper is now quite unresponsive to external stimuli and so is difficult to wake. Heart rate, blood pressure and body temperature continue to drop.
Stage 4: This is the deepest stage of sleep and it is very difficult to wake someone up.
Rapid Eye Movement or REM Sleep: Even though this stage is characterised by brain activity similar to a person who is awake, the person is even more difficult to awaken than stage 4. It is therefore also known as paradoxical sleep. Muscles become totally relaxed and the eyes rapidly dart back and forth under the eyelids.
Stages 1 to 4 are collectively known as non-REM (NREM) sleep. Stages 2-4 are categorised as slow-wave sleep and appear to relate to body restoration, whereas REM sleep seems to aid the strengthening and organisation of memories. Sleep deprivation experiments suggest that if a person is deprived of stage 1-4 sleep or REM sleep he will show rebound effects. This means that in subsequent sleep, he will make up the deficit in that particular type of sleep. This shows the importance of both types of sleep.
As can be seen from Figure 1, sleep occurs in cycles. Typically, the first REM sleep will occur about 90 minutes after the onset of sleep. The cycle of stage 1 to 4 sleep and REM sleep repeats during the night about every 90 minutes. Most deep sleep occurs earlier in the night and REM sleep becomes greater as the night goes on.

Figure 1 Typical cycle of stage 1-4 (NREM) sleep and REM sleep in the course of a night

4.2 Circadian Rhythms
Apart from the alternation between wakefulness and sleep, man has other internal cycles, such as body temperature and hunger/eating. These are known as circadian rhythms as they run on an approximately daily basis. Circadian rhythms are physiological and behavioural functions and processes in the body that have a regular cycle of approximately a day (actually about 25 hours in man).
Although, circadian rhythms are controlled by the brain, they are influenced and synchronised by external (environmental) factors such as light. An example of disrupting circadian rhythms is by taking a flight that crosses time zones. This will interfere with the normal synchronisation with the light and dark (day/night). This throws out the natural link between daylight and the body’s internal clock, causing jet lag, resulting in sleepiness during the day, etc. Eventually however, the circadian rhythm readjusts to the revised environmental cues.
　
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