From NatGeo, "While We Sleep, Our Mind Goes on an Amazing Journey"

"But when we’re sleeping, and we commence our first REM session, the most elaborate and complex instrument known in the universe is free to do what it wishes. It self-activates. It dreams. This, one could say, is the playtime of the brain. Some sleep theorists postulate that REM sleep is when we are our most intelligent, insightful, creative, and free. It’s when we truly come alive. “REM sleep may be the thing that makes us the most human, both for what it does for the brain and body, and for the sheer experience of it,” says Michael Perlis.

Maybe, then, we’ve been asking the wrong question about sleep, ever since Aristotle. The real wonder isn’t why we sleep. It’s why, with such an incredible alternative available, do we bother to stay awake?

And the answer might be that we need to attend to the basics of life—the eating and mating and fighting—only to ensure that the body is fully ready for sleep."