单选题 There is no creature that does not need sleep or complete rest every day.
If you want to know why, just try going without sleep for a long period of time. You will discover that your mind and body would become too tired to work properly. You would become irritable and find it hard to think clearly or concentrate on your work. So sleep is quite simply the time when the cells of your body recover from the work of the day and build up supplies of energy for the next period of activity.
One of the things we all know about sleep is that we are unconscious in sleep. We do not know what is going on around us. But that doesn' t mean the body stops all activity. The important organs continue to work during sleep, but most of the body functions are slowed down.
For example, our breathing becomes slower and deeper. The heart beats more slowly, and the blood pressure is lower. Our arms and legs become limp(柔软的) and muscles are at rest. It would be impossible for our body to relax to such an extent if we were awake. So sleep does for us what the most quiet rest can not do.
Your body temperature becomes lower when you are asleep, which is the reason people go to sleep under some kind of covers. And even though you are unconscious, many of your reflexes(反射 动作) still work. For instance, if someone tickles(使觉得痒) your foot, you will put it away in your sleep, or even brush a fly from your forehead. You do these things without knowing it.

单选题 If you don' t have as much sleep as your body needs, you will ______.
A. work properly B. think clearly
C. keep your attention on your work D. easily get angry

单选题 The cells of your body develop supplies of energy ______.
A. when you are asleep B. when you recover from your work of the day
C. in the next period of activity D. when you are quiet

单选题 In the clause "...that we are unconscious in sleep" ,the word "unconscious" means ______.
A. untiring B. unmoved
C. quiet D. not knowing what is happening around

单选题 When you are sleeping,______.
A. all of you reflexes stop working B. most of your reflexes stop working
C. many of your reflexes still work D. all of your reflexes still work