ABSTRACT
L IFE IS DOMINA TED BY a da ily drive to fall asleep. Such rhyth-micity is characteristic of animal and even plant activity. While the daily cycle of sleeping and waking is the most obvious rhythm for us, there are others, as we shall see. The importance to psychology of such rhythms is that they involve variations in motivated behaviour, emotional state and cognitive performance. For psychobiology the questions are, for example, what are the origins of these cydes? Are they driven by an internaI dock? How are they influenced by external events? What are their implications for daily life? What happens if they go wrang or if we disturb them? Why do we spend about one third of our lives asleep?
