This study 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 the 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.