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.