Sufficient good-quality sleep is one of the mainstays of a healthy lifestyle. What ‘good sleep’ means, however, is not an easy question to answer, for one thing because we do not know what the precise function of sleep is. The assumption is that it is involved in maintaining neuronal networks, including strengthening and consolidating memory, and also in gene expression. Sleep, then, is not just a period of rest but also an active process that depends on sleep pressure (Process S), the biological clock (Process C), age, any underlying illnesses and use of substances and/or medication. Process S is the homeostatic sleep drive: the longer we do not sleep, the stronger that pressure for sleep becomes.Our sleep pattern changes during life. Polysomnography is providing a growing understanding of what happens during sleep and what can go wrong. There are four types of sleep disorder: hypersomnia, insomnia, sleep-related behaviour and movement disorders, and lastly delayed sleep phase syndrome. Many patients with neurological conditions suffer from sleep disorders, which may precede the illness itself.This chapter begins by discussing the physiology of sleep (sect. 19.1). We then explain how sleep diagnosis is carried out (sect. 19.2). The third section deals with the aforementioned four types of sleep disorder (sect. 19.3), and the chapter closes with a brief overview of neurological disorders in which sleep disorders play an important role (sect. 19.4).
