The 24-hour sleep-wake schedules of 51 individuals (age 3 to 21 years) with mental retardation and severe behavior disorders were recorded using a direct-observation data collection system over an average period of approximately one month. The patients in the study had significantly less total sleep and less night sleep than their peers of the same age, and 88% had disturbances of sleep: delays in getting to sleep, frequent night waking or early waking. 'Appropriate' sleep, a measure of the amount and regularity of sleep, correlated positively with standardized measures of IQ and expressive language. 'Total' sleep, that is, the overall number of hours, was not correlated with measures of cognitive functioning. The findings are compared with those from other studies describing the sleep of individuals with mental retardation.