Introduction

Where much has been written about sleep from a clinically disordered perspective, only relatively recently, with the direct and indirect costs of an increasingly demanding 24-hour society being realized, has the concept of sleep entered the arenas of health psychology and behavioural medicine. Moreover, the reported neglect of discussions regarding sleep, beyond the use of hypnotics within doctor–patient interactions, coupled with an increasing understanding of sleep as a modifiable health behaviour has led to calls for sleep being placed high on the health psychologists' agenda. To this end, researchers have uncovered reciprocal links between the quality, quantity and timing of sleep, as both a subjective and objective phenomenon, and numerous health and social outcomes. This chapter explores those links, focusing on the relationship between sleep need, sleep regulation and circadian rhythmicity, whilst examining the influence of human behaviour on these relationships.

What is sleep?

Sleep is an active process consisting of two stages, rapid eye movement sleep (REM) and non rapid eye movement sleep (NREM) (of which there are four sub-stages). Although not discrete, each stage of sleep can be characterized physiologically by different frequencies and amplitudes in brain-wave activity and together they make up what is known as sleep architecture. In Stage 1 the body enters a transitional period between sleep and wakefulness from which awakening is easy.