| 摘 要： | After a busy day of work and play, the body needs to rest Sleep is necessary for good health. During this time, the body recovers from the activities of the previous day The rest that you get while sleeping enables your body to prepare itself for the next day There are four levels of sleep, each being a little deeper than the one before As you sleep, your muscles relax little by little. Your heart beats more slowly, and your brain slows down. After you reaeh the fourth level, your body shifts back and forth from one level of sleep to the other. Although your mind slows down, from time to time you will dream Scientists
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