When it is dark, it is best to save energy and avoid getting eaten. Also to avoid falling off a cliff. Why sleep at all, it seems like a waste of time. Nevertheless, sleep appears to be necessary. There is a debate going on about why we sleep.
Sleep follows a regular cycle each night.
The EEG pattern changes in a predictable way several times during a single period of sleep. A dream is a story that a person watches or appears to take part in during sleep. Dreams are imaginary but are related to events in the dreamer's life. They seem real while taking place. Some dreams are nice and some are scary, called a nightmare.