What is the passage mainly about? [br] [originaltext] In the past 20 years,

In the past 20 years, scientists have learned a great deal about sleep and dreams. They have discovered, for example, that sleep and dreams affect the way people feel. A good night’s sleep, generally, makes a person happier in the morning.
     Dreams also have a strong effect on the moods of people. However, who appears in a dream is more important than what happens. Every person has a special dream character. If this special character appears in dreams, people are happier after sleeping. If there are many people in one’s dreams, this has a good effect. A person will wake up happier.
    The moods of people affect their performance throughout the day. The level of one’s moods rises and fails during the day. The less sleepy people are, the better they perform. And, usually they are more friendly, more aggressive, and more clear-thinking. Someday, scientists hope to be able to manipulate sleep in order to control the way people feel and perform.