[填空题]Everyone needs to sleep. If we don’t sleep enough, we often feel tired and irritable (烦躁的，易怒的). It seems there are two purposes 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 dreaming occurs. In passive sleep, the body is at rest. The heartbeat slows down. The body processes become very slow. We move very little. And the brain becomes very inactive. If a person continues to sleep, she or he enters a new stage, a more active stage. The body goes through several changes: the brain temperature rises, the amount of blood in brain increases, the body becomes very still and the brain becomes active, the eyes begin to move rapidly, and the person is dreaming.
Throughout the night, people alternate between passive and active sleep. During eight hours of sleep, people dream for a total of one and a half hours on the average.
Many people say. that they don’t dream, or that they rarely dream