Sleep problems associated with autism spectrum disorders (ASD) have been well documented, but less is known about the effects of sleep problems on day-time cognitive and adaptive performance in this population. Children diagnosed with autism or pervasive developmental disorder-not otherwise specified (PDD-NOS) (N = 335) from 1 to 10 years of age (M = 5.5 years) were evaluated for the relationships of Behavioral Evaluation of Disorders of Sleep (BEDS; Schreck, 1998) scores to measures of intelligence and adaptive behavior. Results suggested that children who slept fewer hours per night had "lower" overall intelligence, verbal skills, overall adaptive functioning, daily living skills, socialization skills, and motor development. Children who slept fewer hours at night with waking during the night had more communication problems. Breathing related sleep problems and fewer hours of sleep related most often to problems with perceptual tasks. The results indicate that quality of sleep--especially sleep duration--may be related to problems with day-time cognitive and adaptive functioning in children with autism and PDD-NOS. However, future research must be conducted to further understand these relationships. (Contains 5 tables.)