Bet no one knows what sleep really is. Once we get beyond the simple level of recognizing a superficially quiet state that is very different from wakefulness, we find that the major questions about the nature of sleep are largely unanswered. These questions are: 1. What happens to us while we sleep? At a time before sleep was studied scientifically, laymen and intellectuals alike thought they knew the answer, i.e., that sleep is a state in which “everything simply slowed down or stopped.” That answer is wrong. Although our bodies move less and we make less noise while we sleep, there are many complex changes in our brains and minds during sleep which cannot be described as a simple reduction of activity. For example, there are nerve cells in our brains which fire 5 to 10 times more frequently during certain sleep stages than during wakefulness. 2. Why do we sleep?