No one knows why we sleep, but it's

No one knows why we sleep, but it's certain that we need to. People who are prevented from sleeping begin to suffer obvious effect after a few days一they think less clearly, and they fall asleep during the working hours. Some may have hallucinations (幻觉). There are no rules about sleep. Generally speaking, grown-ups sleep about seven 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 five 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一- 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一