We will examine whether sleep problems in children with autism spectrum disorder (ASD) are related to alterations in the production of melatonin (MT), a hormone that plays an important role in regulating sleep-wake cycle. Furthermore, we will examine whether MT is effective in improving sleep in children with ASD. Children with ASD experience high rates of sleep disturbances that potentially contribute to problems with thinking and behavior. It is unclear if changes in MT production cause sleep problems in children with ASD. MT is frequently used to treat these sleep problems; however, it has not been well established whether MT is an effective treatment. Our hypotheses concerning MT are: 1. Children with ASD and sleep problems will have a delayed sleep-wake cycle and/or decreased MT production; 2. Treatment with MT will be associated with improved sleep and behavior. All subjects will be recruited from one of three sites: Baylor College of Medicine, Oregon Health & Science University and Columbia University. We will use a standardized questionnaire to determine whether the child has sleep problems. We will measure MT levels in saliva in ASD children with sleep problems and in a group of ASD children without sleep problems. Total 24-hour MT production will be determined from urine samples in these same two groups. Eligible participants will then be enrolled in a randomized trial of three oral doses of MT (3, 6, 9 mg) and a placebo. Neither children nor investigators will know which doses are being given to which participants. We will determine whether treatment with MT helps children fall asleep faster and whether their behavior improves, as reported on standardized questionnaires. Results from this study will inform the development of future trials of sleep-wake interventions and clinical guidelines for the use of MT to manage sleep problems in ASD.