Sleep and dreaming are manifestations in higher organisms of a fundamental 'circadian rhythm' of inactivity-activity. During the past thirty years, research has provided a great deal of new information about the phenomenom and phenomenology of sleep, and the relationship between sleep and wakefulness. This book aims to describe, organise and interpret some of this new knowledge in order to stimulate a greater appreciation of the role of sleep and dreaming in human adaptation. The study of sleep and dreaming provides a very special perspective on human functioning. It stands in direct contrast to more traditional paradigms utilised in psychology that place the locus of explanation of human behaviour in the 'external environment'