Sleep is a state of physical inactivity and mental rest in which conscious awareness, thought, and voluntary movement cease and intermittent dreaming takes place. This natural and regular phenomenon essential to all living creatures normally happens with the eyes closed and is divided into two basic types: REM (rapid eye movement) and NREM (non-rapid eye movement) sleep. As passive as sleep appears, it is actually a very active and deliberate process in which the brain busily turns off wakeful functions while turning on sleep mechanisms. No one knows exactly why we must sleep or how it happens, but the quality, quantity, and type of sleep affects the quality, quantity, and effectiveness of our wakeful mental and physical activities. These, in turn, influence the quality, quantity, and timing of sleep.

In the attempt to understand our need for sleep, experiments in sleep deprivation play an important role. Total sleep...