This remarkable book brings together vast amounts of information and knowledge about a question that has always captured man's imagination: Why do we sleep? The author begins with a pungent review of historical and contemporary theories of the functions of sleep and of recent research in sleep deprivation and synchronized (non-dreaming) and desynchronized (dreaming) sleep. Citing his own studies, he investigates the reasons for variable sleep patterns and finds that sleep requirements are influenced by differences in personality, as well as age, life style, and mental state. He then explores the effects on sleep of psychological stress, physical and intellectual activity, and the use of drugs and of other chemicals. The different kinds of tiredness and the role of dreaming in sleep function are also studied. Written clearly enough for the layman to understand, this book is nevertheless a sophisticated theoretical contribution to the literature on sleep. Hartmann combines the rigor of the laboratory scientist with the sensitivity of the clinician. His conclusions offer some fascinating controversial notes on the relationship between the brain and the mind.