单选题
No one knows exactly why we sleep. But scientists have learned a lot about how we sleep.
When you first fall asleep, your heart starts to beat more slowly and your breathing slows down. If something wakes you, you might not think that you had been asleep. Some people call it dozing (打盹). Next, you enter into a deeper sleep. Your pulse (脉搏) and breathing become even slower. But you can still be awakened quite easily. If you take a catnap, you probably will not get any further than this stage (阶段) of sleep. If you sleep longer than about twenty minutes, you get into the third one. Your body is very relaxed. It would take a loud noise to wake you up. You may have heard of people who walk in their sleep. No one knows what makes people sleepwalk. But a person can sleepwalk only during stage four. This is the last and deepest kind of sleep. If someone wakes you up, you might feel very uncomfortable and you need a few minutes to get used to being awake. After about ten minutes of this stage sleep, you go back to stage three and then to stage two. Then something quite different begins to happen. Your heartbeat becomes rapid. Your eyelids flutter (颤动), and your eyes move. This stage of sleep is called REM (rapid eye movement) sleep.
Most dreams happen during REM sleep. Scientists who study dreams often make noises or talk to sleeping people in the REM stage. They are trying to find out what effects noises can have on dreams, what causes dreams and how dreams might be helpful to people.

单选题 The sleep experiment show that there are ______ different stages of sleep.

单选题 When a person is seen dozing, it suggests that he ______.

单选题 The following phenomena might happen when one sleeps. Which happens last during his non-REM sleep?

单选题 When a dream happens, the right order of stages of sleep is ______.