What’s the topic of the passage?

No one knows exactly why we sleep. But scientists have learned a lot about how we sleep. From sleep experiments, scientists have learned that there are four different stages of 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 this kind of sleep dozing. Scientists call it stage one sleep.
    Next, if you are not awakened, you drift into a deeper sleep. Your pulse and breathing become even slower than they were during stage one sleep. But you can still be awakened quite easily. If you take a cat nap, you probably will not get any further than this stage of sleep, which is called stage two.
    If you sleep longer than about twenty minutes, you go into the third stage of sleep. Your body is very relaxed. It would take a loud noise to wake you up.
    You have probably 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 deepest kind of sleep. If someone wakes you up during stage four, you might feel very confused. It might take you a few minutes to get used to being awake.