The advances in research on sleep an biological rhythms have recently been applied to the diagnosis and treatment of sleep disorders. A new clinical specialty has developed with the establishment of sleep disorder centers and a diagnostic classification of sleep and arousal disorders. This new nosological approach has evolved from an extensive base of new scientific information concerning descriptive polygraphic and analysis of clinical case series. Four major categories have been defined: (a) disorders of initiating and maintaining sleep (insomnias), (b) disorders of excessive somnolence, (c) disorders of the sleep-wake schedule, and (d) dysfunctions associated with sleep. Within this comprehensive classification certain major pathophysiological advances are described for the "insomnias." These include polysomnographic identification of altered sleep stage patterns in the major effective illnesses, insomnias related to hypnotic drugs and alcohol, sleep disturbances associated with sleep-induced respiratory impairment, and sleep-related periodic movements during sleep (nocturnal myoclonus). Excessive daytime somnolence is primarily associated with the hypersomnia sleep-apnea syndrome and with narcolepsy. The relationship between biological rhythms (chronobiology) and disorders of the human sleep-wake schedules is very actively investigated. The recognition that sleep length, internal organization, and timing within neurophysiological circadian time-keeping systems has lead to better diagnosis of these sleep-wake disorders and new chronotherapeutic regimens. Finally, increasing identification and description of "parasomnias," i.e. dysfunctions associated with sleep, has led sleep research into important new areas that are of general physiological interest. It is now clear that sleep disorders medicine has become a new scientific and clinical discipline in its own right.