Study flow diagram. The study will recruit 99 participants from Alberta Children’s Hospital Emergency Department and surrounding sports medicine and pediatric centers. Participants are randomized after baseline data collection to melatonin 3 mg, melatonin 10 mg, or placebo in a 1:1:1 ratio. There is a four-week treatment phase. Baseline assessments (day 30 +/-10 days) include standard history and clinical assessment, neurocognitive assessment, CHQ, BASC-2, BRIEF and actigraphy. During treatment the participant will keep daily sleep and treatment logs. Actigraphy will continue through treatment. Weekly telephone calls will be made to monitor adverse effects. Post-treatment assessments occur during days 59 to 70 and a final telephone follow-up will occur at day 90 (+/-7 days). CHQ (Child Health Questionnaire), BASC-2 (Behavioral assessment system for children), BRIEF (Behavior Rating Inventory of Executive Function).