There are two fundamental theories as to why sleep occurs:
Sleeping is part of the circadian rhythm, where the body functions on an approximate 24-hour cycle. Exactly what effect sleep has on the cycle is not yet agreed upon however.
Sleep is in response to fatigue created by the day’s wear and tear on the body, and while the person sleeps, the body makes physical repairs to itself.
If clinical studies are a useful way to assess the importance of sleep then the circadian rhythm theorists seem to be winning at the moment because:
Nearly all bodily functions continue regardless of whether the person is asleep or awake.
If people are allowed to sleep and wake whenever they like, they usually do so around the same time each day.
Even when going to bed late, people still wake up around their usual time.
If sleep was needed to repair the body, then it would be natural for a person to sleep for longer periods while under extra physical stress, yet this is not the case.
While muscles require periods of rest, they do not seem to require sleep, because people who have had their spinal cord severed at the neck do not suffer any damage to their body by the fact that the brain cannot tell it to sleep.
The fatigue theory should not be written off however, because there are lots of examples how health is assisted or restored during sleep. For example, in the stages 3 and 4 of the sleep cycle, the cardiovascular system has a rest because heart rate and blood pressure gradually drop. Even though there are boosts to this during REM sleep, overall heart rate and blood pressure are approximately 10% lower than during wakefulness.
The volume of air breathed is on average around 20% lower than when resting but awake, so the respiratory system also has a bit of a rest during sleep.
When the level of cytokines, which are proteins that act as messengers between cells and help to regulate the body’s immune response to disease or fighting germs, are increased, the person is prone to feeling sleepy. Presumably this allows the body to use more energy in the healing process instead of busying itself with other work.
If extra body-repairing is the prime purpose of sleep then it is possible that it occurs during stage four of the sleep cycle. This is because when a person is sleep deprived, the body automatically spends more in the non-REM phases, and especially in stage four.
Sleeping appears to help certain types of memory because studies have shown that even a relatively short nap seems to help people with repetitive skills and problem solving. It is possible then that one of the reasons for sleeping is to allow ready recall of memories.
The most popular reason for wanting to go to sleep for practically anyone is simply because of feeling tired. The result of a sound sleep is a feeling of being refreshed, or a lack of fatigue, so perhaps the reason for sleep is a mixture of the need to maintain a circadian rhythm and also helping the body to have some time out when it is really needed.