No one knows why we sleep, but it's certain that we need to. People who are prevented(阻止) from sleeping begin to suffer obvious(明显的) effects after a few days—they think less clearly, and they fall asleep during the working hours.
There are no rules about sleep. Generally speaking, grown-ups sleep about 7 and a half hours each night and probably more than 60 percent get between seven and eight hours. But perhaps eight percent are quite happy with 5 hours or less, and four percent or so find that they want ten hours or more. If you feel all right, you're probably getting enough sleep. The important thing is not to worry how much other people get—their needs may be different. Exercise doesn't seem to increase the need for sleep—office workers, for example, sleep for about as long as people doing physically active work.
Children sleep more than grown-ups—perhaps 14 to 18 hours soon after birth, going down to grown-up levels by early teenage. Sleep patterns also tend (倾向) to be different in the old people, who may sleep less at night than they did when younger, find sleep getting more broken, and often make it a rule to sleep during the daytime.
