Sleep is something we all do. But some people need to sleep more than others. Babies sleep most of the time. Children in school sleep about ten to twelve hours a night. Most grown-ups sleep only seven or eight hours.     
All parts of our bodies have to rest after they work. Our arms need a rest after we lift heavy things. When we run fast, our legs work hard. They get tired. We have to rest them. Our brains work hard, too.    72     We can sit quite still and rest our arms and legs. But our brains aren’t resting. They go right on thinking as long as we are awake.
Our brains slow down a bit when we sleep and dream.   Instead of thinking wide-awake thought, our brains make up dreams. Some dreams are very pleasing. Some are not. Most of the time we forget them when we wake up.
Scientists have tried to find out what would happen if people were not allowed to sleep. They asked some people not to go to bed. The people stayed up all night and all the next day. They stayed up the next night, too and the day after. They played games. But they made mistakes. They forgot things.    . The people grew rude and mean. They became angry with their friends. Finally they were too tired to stand up. The moment they sat down, they fell asleep.
Scientists found that if people are not allowed to sleep, they act in an unusual way.
     But we do know that we need it to stay well.
So tonight have a good sleep. Lie down under the covers. Shut your eyes. Let your thoughts travel about. Soon you’ll stop thinking. You’ll be asleep.

A．But even as we sleep our brains are doing some work.

B．It was hard for their tired brains to work.

C．When we are awake, they help us pay attention to the world around us.

D．No one knows quite surely why sleep is so good for us.

E. Good sleep helps to improve one’s memory.
F. But babies, children and grown-ups ----all of us need to have our sleep.