STUDY OBJECTIVES: To perform a meta-analysis of the efficacy and safety of exogenous melatonin in advancing sleep-wake rhythm in patients with delayed sleep phase disorder. DESIGN: Meta analysis of papers indexed for PubMed, Embase, and the abstracts of sleep and chronobiologic societies (1990-2009). PATIENTS: Individuals with delayed sleep phase disorder. INTERVENTIONS: Administration of melatonin. MEASUREMENTS AND RESULTS: A meta-analysis of data of randomized controlled trials involving individuals with delayed sleep phase disorder that were published in English, compared melatonin with placebo, and reported 1 or more of the following: endogenous melatonin onset, clock hour of sleep onset, wake-up time, sleep-onset latency, and total sleep time. The 5 trials including 91 adults and 4 trials including 226 children showed that melatonin treatment advanced mean endogenous melatonin onset by 1.18 hours (95% confidence interval [CI]: 0.89-1.48 h) and clock hour of sleep onset by 0.67 hours (95% CI: 0.45-0.89 h). Melatonin decreased sleep-onset latency by 23.27 minutes (95% CI: 4.83 -41.72 min). The wake-up time and total sleep time did not change significantly. CONCLUSIONS: Melatonin is effective in advancing sleep-wake rhythm and endogenous melatonin rhythm in delayed sleep phase disorder.