When did the scientists learn a great deal about sleep and dreams? In_________

In the past 20 years scientists have learned a great deal about sleep and dreams. They’ve discovered, for example, sleep and dreams affect the way people feel. A good night’s sleep generally makes the person happier in the morning, which will affect his performance throughout the day. So scientists hope to be able to control sleep in order to control the way people feel and perform.
    It is likely that such animals as dogs, cats, and monkeys dream, too. In a study, monkeys were trained to push a lever whenever they saw a picture on a screen. At different times during their sleep the monkeys suddenly began to push the lever as if responding to an observed image.