This book examines the purpose of sleep in mammals, particularly in humans. The author's approach takes a broad biological perspective, clearing away many misconceptions in order to derive a number of well-founded hypotheses about a variety of aspects of sleep. Technicalities are kept to a minimum, and an effort has been made to make the book understandable to a wide readership, including anyone with a basic background in biology and psychology. The emphasis is on the role of sleep itself, not on the physiological mechanisms that assist its occurrence.