For millennia it was believed that sleep was merely a passive state in which the body and mind rested or, as Aristotle believed, just an unremarkable and unimportant period marked by an absence of our usual sense perceptions. As often as not, it was thought of as a necessary evil or, at best, a kind of envelope or container for the more interesting and important act of dreaming.
However, it has become abundantly clear in recent decades is that sleep is not simply a matter of putting our mental and physical activities on hold for a while, but rather a genuine “second state” with its own complex and varied mental and physical activities. Although we do not have a definitive answer to the question “why do we sleep?”, with the recent increases in knowledge about exactly how sleep works (see the section on How Sleep Works), we are at least now in better position to hazard some more educated guesses about just why we sleep.
Sleep appears to be an essential physiological process for humans and for most other animals, other than very simple ones with small brains. When deprived of sleep, we function less effectively, feel tired and irritable, make more mistakes, are less creative and, if taken to extremes, ultimately die. In the same way as a feeling of hunger reminds us of the basic human need to eat, a feeling of sleepiness reminds us of our essential need to sleep.
Exactly how or why sleep is essential, though, is more difficult to pin down. As William Dement, co-discoverer of REM sleep and pioneering sleep researcher, puts it: “As far as I know, the only reason we need to sleep that is really really solid is because we get sleepy.”
Over the years, many theories about the function of sleep have been put forward, several of which are briefly described in this section. The actual reason we sleep is likely to be a combination of most or all of them.