Sleep difficulties in children with autism spectrum disorders (ASD) are common reasons why parents seek medical intervention for their children. Identifying a safe and effective pharmacologic agent that promotes sleep in ASD would favorably impact the lives of these children and their families. In this study we plan to determine the dose-response, tolerability and any adverse effects of supplemental melatonin in 30 children with ASD.The melatonin dose levels are 1mg, 3mg, 6mg, and 9mg. After a 3 week baseline period, the child will begin melatonin at 1mg and will dose escalate every three weeks until he/she is falling asleep within 30 minutes of bedtime at least 5/7 nights per week. No child will take more than 9 mg of supplemental melatonin.