Sleep disturbance and insufficiency are associated with physical and mental health problems in school-aged adolescent children. Early start times (before 8:30 a.m.) restrict sleep for adolescents at over 80% of US middle and high schools. Delaying school start times could be an effective population-level strategy to promote adolescent health and well-being. We review the evidence for the effect of school start time on adolescent sleep, academic achievement and truancy, and mental and physical health. Consistent evidence shows that adolescents at later-starting schools sleep longer than those at earlier-starting schools. Limited evidence suggests that students at later-starting schools may learn more, attend school more regularly, have fewer mental health problems, and are less prone to unintentional injury. We conclude by discussing the trade-offs school districts weigh when considering scheduling changes, and the need for school start time research to improve study design and expand the scope of outcomes.