ABSTRACT
Scientific explorations of the nature and function of sleep have concentrated on its electrophysiology. This chapter provides a much broader perspective by acknowledging at least three quite different aspects of sleep: experience, behaviour and physiology. Sleep-related experiences do not occur only while we are asleep but begin long before and persist long after the sleep period. Drowsiness is the most obvious phenomenon. The chapter shows how a comparative, developmental and evolutionary study of sleep may assist in the understanding of its presence in man. Sleep deprivation usually, but not always, results in a rebound increase in whatever was deprived when normal sleep conditions are resumed. It is almost true that every brain structure is affected by the transition from waking to sleep and from one sleep state to another. Although energy consumption during sleep is considerably lower than during active wakefulness, it is only slightly lower than during relaxed wakefulness.
