Recent scientific explorations of the nature and function of sleep have concentrated on its electrophysiology. Whilst this has proved to be an exciting and profitable exercise, this review will attempt a much broader perspective by acknowledging at least three quite different aspects of sleep: experience, behaviour and physiology. After all, sleep is not only found in the physiology laboratory: it is something which we all experience regularly and something which we see other people and other animals doing every day. We look to science for a better understanding of why we do it and what happens while we are doing it and therefore the success of the scientific endeavour must be judged by the light it throws on our everyday experience of a very common phenomenon. In particular, sleep appears to have a long evolutionary history and one of the purposes of this chapter is to show how a comparative, developmental and evolutionary study of sleep may assist in our understanding of its presence in man.