What is the main topic of this talk?

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