“In short most of us think of sleep as a passive and relatively constant and unchanging process. In fact, sleep is a very active state. Our bodies move frequently, as we roll about during the night, and, more importantly to the psychologist, our brain activity is even more varied than it is during the normal waking state.”

• Dement, W.C. (1978). Some must watch while some must sleep. New York: W.W. Norton.
• Pinel, J.P.J. (1992). Biopsychology. Needham Heights, MA: Allyn & Bacon.