People may believe sleep to be simply a static state that is the direct opposite of wakefulness; however, this is not the case. Rather, it is a complex and dynamic process, and throughout sleep we progress through multiple stages that can be measured discretely across behavioural, physiological, and cognitive domains. This chapter describes the differences and features of these different stages and how they can be measured. Also described is the fact that sleep and wakefulness are not mutually exclusive, and that there are times when the lines between sleep and wake can be blurred, and this is notably true in insomnia. Finally, the chapter explains how sleep is regulated through interacting homeostatic and circadian processes, and the neuroscientific underpinnings of the sleep and circadian system.

Sleep is one of the great mysteries of human experience. It is a behaviour that is highly preserved throughout the animal kingdom, and it constitutes a third of our existence. It begins before birth and is with us throughout life. For a newborn baby, it is their predominant behaviour and continues to occupy a large proportion of our lives as we age. It has long been suspected that it is essential for good mental and physical health, and in the last century, there has been growing scientific evidence to confirm this suspicion. Yet, we know very little about the functions of sleep and are only just beginning to understand the mechanisms that control it. In the past, it was commonly assumed that sleep had ’a function’, but the more we have come to understand, the clearer it has become that sleep performs multiple functions. Precisely what those functions are is still uncertain, and they may change across the lifespan or possibly even vary across different species. We know that sleep is essential for life and total sleep deprivation for long enough is fatal, and the fact that we spend a third of our lives in this vulnerable state underlines how important sleep must be.