cognitive
The brain still is in use even though it is asleep. This is why we dream because the brain is imagining all of the possibilities in the world and puts it into your mind all way you are asleep. The body repairs cells, restores energy, and releases molecules like hormones and proteins. There have been several studies over this fact and none have come to a definitive conclusion, but there are many theories. Dreams are used to coordinate emotions, such as coping with fear. Integrating memories by repeating things from your day to remember them. To solve real problems, or on the other hand, to forget them.
“I think sleeping was my problem in school. If school had started at four in the afternoon, I’d be a college graduate today.” – George Foreman