There is no creature that does 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 or 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 cannot 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）our foot, you will put it away in your sleep, or even brush a fly from your forehead. You do these without knowing it.