Sleep and sleep disorders: an overview.

Sleep has been preserved during evolutionary history and is found in all mammalian species and birds. Despite the apparent strong survival advantage of sleep, and the observation in experimental animals that persistent sleep deprivation leads to rapid physical deterioration and death, the function of sleep is poorly understood. Modern neurophysiologic techniques have shown that sleep is not a homogeneous state but rather is divided into rapid eye movement and non-rapid eye movement stages. These two sleep states are as different from one another as sleep is from wakefulness. The advent of polysomnography has led to the recognition of a variety of sleep disorders; some, such as sleep apnoea, have potentially fatal consequences. Bruxism is an example of a parasomnia, viz a disorder which intrudes in normal sleep but does not lead to a primary sleep complaint. Recent improvements in diagnostic and therapeutic approaches to sleep disorders has led to the development of multidisciplinary Sleep Disorders Clinics.