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We spend about a third of every day asleep, often in continuous eight-hour stretches - although many cultures have different habits, and there is evidence that at certain times in history, nightly sleep was routinely taken in two chunks. Whatever the pattern of sleep and wakefulness, however, it is usually regular. We appear to have an internal body clock that regulates this circadian rhythm.
In 1962, French cave explorer Michel Siffre spent two months underground with no contact with the surface, and discovered that he naturally fell into a 25-hour daily pattern. It is clear that we need to sleep on a regular basis and disturbing this daily rhythm is harmful physically and psychologically - jet lag, recurrent illness among shift workers and the use of sleep deprivation as a means of torture amply demonstrate these effects. However, psychologists disagree the purpose of sleep, whether it is physically or psychologically restorative, or serves some other evolutionary function.