It`s clear that everyone needs to sleep. Most people rarely think about how and why they sleep, however. We know that if we sleep well, we feel rested. If we don`t sleep enough, we often feel tired and irritable.
It seems there are two purposes of sleep: physical rest and emotional and psychological rest. We need to rest our bodies and minds. Both are important in order for us to be healthy. Each night we alternate between two kinds of sleep: active sleep and passive sleep. The passive sleep gives our body the rest that`s needed and prepares us for active sleep, in which dreaming occurs.
Throughout the night, people alternate between passive and active sleep. The brain rests, then it becomes active, then dreaming occurs. The cycle is repeated: the brain rests, then it becomes active, then dreaming occurs. This cycle is repeated several times throughout the night. During eight hours of sleep, people dream for a total of one and half hours on the average.
很明白，人人都需要睡眠。然而，大多数人很少思考他们是如何或为何睡眠的`。我们知道只要睡好了，就觉得休息好了。如果睡眠不足，就会觉得乏困，易怒。
睡眠好像有两个目的：身体休息和情绪或心理休息。我们需要让躯体和大脑都得到休息。为了保持健康，二者都很重要。每天夜里，我们交替在两种睡眠之间：积极睡眠和消极睡眠。消极睡眠使我们的身体得到所需的休息，同时为积极睡眠做准备。在积极睡眠中，梦就出现了。
整个晚上，人们在消极睡眠和积极睡眠间变换。大脑得以休息，随后活跃起来，接着梦便开始。如此循环：大脑休息好了，活跃起来，梦又开始了。这样的循环整个晚上要反复好几次。八小时的睡眠中，人们平均要做上整整一个半小时的梦。
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