It is clear that we all need to sleep. Most people rarely think about how or why they sleep, however. We know that if we sleep well, we feel rested. If we do not sleep enough. we often feel tired and irritable. It seems there are two purposes of sleep: physical rest and emotional, or psychological, rest. We need to rest our bodies and our minds.

Each night we alternate between two kinds of sleep: active sleep and passive sleep. 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 heart beat slows down, and the body processes become very slow. We move very little, and the brain becomes very inactive. This stage of sleep is known as slow-wave sleep because the brain waves move in a very slow, regular rhythm.

显然我们都需要睡眠。大多数人很少考虑他们是如何睡眠的,也很少考虑他们为何要睡眠。我们知道,如果睡得好,我们就会感到有了充分的休息;如果睡眠不充足，我们常会感到疲倦和烦躁。看来睡眠有两个目的:一是为了获得体力上的休息，一是为了得到精神上的休息(或叫做心理休息)。我们需要让自己的身体和大脑都得到休息。

每天晚上我们以两种交替出现的方式睡眠,即积极睡眠和消极睡眠。消极睡眠给我们的身体以所需要的休息,并让我们向积极睡眠过渡。人们在积极睡眠时会做梦。消极睡眠时我们的身体处于一种安宁的状态,我们心脏的跳动减缓，我们人体机能的运行变得非常缓慢。我们身体很少移动,大脑几乎处于静止状态。这一阶段的睡眠叫做慢波睡眠,因为这时脑电波处于一种非常缓慢而有节奏的波动状态。

If we continue to sleep, we will enter a more active stage, in which the body goes through several changes. In this new stage, the brain temperature rises, the amount of blood in the brain increases, the body becomes very still, and the brain goes from being very inactive to being active. As the brain becomes more active, the eyes begin to move rapidly. The eye movement signals that the person is dreaming.

Throughout the night, we alternate between passive and active sleep. The brain rests, then it becomes active, then dreaming occurs. This cycle is repeated several times throughout the night. During eight hours of sleep, we dream for a total of one and a half hours, on the average.

如果我们继续睡眠，我们便会进入一个比较积极的睡眠阶段,这时人体会历经几种变化。在这个睡眠新阶段,大脑温度会上升,大脑血液流量会增加，人体会变得几乎纹丝不动,而大脑则由静止不动转为活动状态。随着大脑活动变得较为积极，人的眼球开始快速运动起来。人的眼球运动表明了我们正在做梦。

整个晚上我们始终处于消极睡眠和积极睡眠两种状态相互交替的过程。大脑保持静态,然后积极活动起来,然后我们开始做梦。这一过程会一夜循环出现好几次。在8小时的睡眠过程中,我们做梦的总时间平均为1个半小时。

We all experience these dream cycles; no one is an exception. Many people say that they do not dream, or that they rarely dream. This is not what actually happens, because everyone dreams and in fact, everyone needs to dream in order to stay healthy. The truth is we need both kinds of sleep: we need passive sleep to rest our bodies and active sleep in order to dream. Dreaming helps us to rest our minds.

人人都会经历这些做梦的过程,无一人例外。许多人说自己不做梦,或者说很少做梦。而实际情况并非如同他们所说的那样,因为每个人都会做梦,事实上人人都需要通过做梦来保持身心健康。实际情况是我们需要两种睡眠:我们需要消极睡眠以休息身体,我们还需要积极睡眠以进入梦乡,而做梦有助于我们的大脑得到休息。

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