Children with autism spectrum disorder (ASD) experience high rates of sleep disturbances that potentially contribute to the severity of their cognitive and behavioral dysfunction and to poor quality of life for themselves and their families. This research will examine whether sleep problems in children with ASD are related to alterations in the secretion of melatonin (MT), a hormone that plays an important role in regulating the circadian cycle and sleep. It will also determine the efficacy of MT for improving sleep in children with ASD. Participants in the study will be recruited from the Autism Treatment Network (ATN) and parents will complete a validated sleep questionnaire to assess presence or absence of sleep problems in their child. Levels of melatonin will be measured before and after a child's usual bedtime and the total 24-hour MT production will be determine via urine samples from each child. In addition, a double-blind, randomized trial of three oral doses of MT (3, 6, 9 mg) and placebo in children ages 4-9 years will establish efficacy of its use in managing sleep problems in children with ASD. Data from this study will provide important information concerning circadian rhythm dysregulation in ASD and will support the development of future studies using melatonin to modify and correct abnormal circadian rhythms.