It's clear that everyone needs to sleep. Most people seldom ．how and why they sleep, however. We know that if we sleep well, we feel rested. If we don't sleep enough , we often.

    It seems there are two purposes of sleep: physical rest and mental （精神的）rest. We need to rest our bodies and minds. Both are important in order . Each night we change between two kinds of sleep: active sleep and passive sleep. The passive sleep gives our body the rest that is needed and prepares us for active sleep, in which dreams occur（发生）.

    people change between passive and active sleep . The brain rests, then it becomes active, then dreaming occurs. The is repeated. This cycle is repeated several times. During eight hours of sleep, people dream for a total of one and half hours.