Background: Melatonin, an endogenous neurohormone with both chronobiotic and antioxidant properties, has been proposed as a nutritional aid for recovery and performance optimization. However, its timing-dependent effects on athletic performance remain unclear. This systematic review and meta-analysis aimed to evaluate the timing-dependent effects of melatonin supplementation on exercise performance and exercise-induced muscle damage in athletes and physically active individuals. Methods: A comprehensive search of PubMed, Web of Science, SPORTDiscus, and Cochrane Library was conducted from inception to September 2025. Only randomized controlled trials (RCTs) published in English examining melatonin versus placebo effects on athletic performance and muscle damage biomarkers were included. Results: In total, 19 RCTs involving 266 participants met the inclusion criteria. Meta-analysis revealed small effects on explosive power (SMD=0.29, 95% CI: 0.05 to 0.53, p=0.02, I2=0%) and moderate effects on endurance performance (SMD=0.58, 95% CI: 0.29 to 0.87, p<0.001, I2=46%). Melatonin significantly reduced creatine kinase levels (SMD=0.59, 95% CI: 0.29 to 0.89, p<0.0001, I2=0%), with non-significant effects on lactate dehydrogenase (SMD=0.45, 95% CI: -0.03 to 0.94, p=0.07, I2=56%). Subgroup analyses revealed timing-dependent effects: evening administration with >6-h exercise intervals produced superior benefits for both endurance performance and explosive power compared to daytime administration or shorter intervals. Higher doses (6-10mg) and athlete populations demonstrated greater improvements, while adolescents (=18years) showed enhanced explosive power responses. Conclusion: These findings suggest that melatonin supplementation, particularly when administered in the evening with adequate timing intervals, enhances endurance performance and reduces exercise-induced muscle damage in athletes during intensive training periods. Systematic review registration: https://www.crd.york.ac.uk/PROSPERO/view/CRD420251178430, Identifier (CRD420251178430).
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