Children with ASD experience high rates of sleep disturbances that potentially contribute to problems with thinking and behavior. It is unclear whether changes in melatonin (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. This study will examine whether sleep problems in children with autism spectrum disorder (ASD) are related to alterations in the production of melatonin, a hormone that plays an important role in regulating sleep-wake cycle. This study will also examine whether MT is effective in improving sleep in children with ASD. The two hypotheses that will be tested are: 1) Children with ASD and sleep problems will have a delayed sleep-wake cycle and/or decreased MT production compared with those without sleep problems; and 2) Treatment with MT will be associated with improved sleep and behavior. Sleep disturbance will be measured using a standardized questionnaire. The MT levels will be measured in saliva in two groups of children with ASD both with and 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 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. This study will also determine whether treatment with MT helps children fall asleep faster and whether their behavior improves. What this means for people with autism: 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.