Exogenous Melatonin and Sleep Quality: A Scoping Review of Systematic Reviews

Melatonin is increasingly used to treat sleep disturbances, yet its overall efficacy remains unclear due to variability in existing evidence. This scoping review aimed to synthesize systematic reviews with meta-analyses assessing the effects of exogenously administered melatonin on sleep quality in humans. Seven databases were searched from inception to July 9, 2025. Eligible studies were systematic reviews containing at least one meta-analysis evaluating melatonin's effects on any domain of sleep quality compared to any comparator. Fifty-seven systematic reviews were included, comprising 227 meta-analyses. Overlap in primary studies was low (corrected covered area = 2.5%), suggesting that reviews drew on largely distinct evidence bases. Methodological quality was variable: only 8.8% of reviews met all seven predefined criteria for rigor, including protocol pre-registration, dual screening, and bias assessments. Vote counting based on the direction of effect was used to summarize efficacy. Of the 215 meta-analyses comparing melatonin to an inactive comparator, 80.9% favored melatonin, 7.9% favored the comparator, and 11.2% reported unclear results. Sleep quality was assessed using heterogeneous definitions and tools, with few reviews evaluating overall sleep quality directly. Adverse events were commonly reported and generally mild, with headaches, gastrointestinal problems, and dizziness most frequently observed. However, inconsistent terminology and reporting limited synthesis. Despite heterogeneity in review methods and outcome definitions, the direction of evidence consistently favored melatonin over placebo. These findings support the feasibility of a future quantitative umbrella review to estimate pooled effects and guide clinical practice.