Effects of morning versus evening exercise on sleep: a systematic review of comparative studies with a meta-analysis of randomized controlled trials.

This systematic review synthesized evidence from comparative studies examining the effects of morning versus evening exercise on sleep, with a meta-analysis restricted to randomized controlled trials. Of the 1111 records identified, 25 studies met the eligibility criteria, including five observational studies, two non-randomized intervention studies, and 18 randomized controlled trials (RCTs), 16 of which were included in the meta-analysis. At the group level, the available evidence does not indicate a clear overall advantage of morning over evening/nighttime exercise for most sleep outcomes. Evening/nighttime exercise was associated with higher wake after sleep onset (WASO) (SMD = -0.51, 95% CI [-0.89, -0.12]), indicating a modest effect on sleep continuity. However, this finding should be interpreted with caution, as potential small-study effects were identified. Subgroup analyses suggested that timing-related differences were more apparent at higher exercise intensities. Overall, these findings support flexible exercise timing that prioritizes adherence and individual constraints, while recognizing that effects vary with intensity, proximity to sleep, and recovery, and that evening moderate-to-vigorous exercise may impair sleep continuity. Future research should identify the exercise-related and individual-level factors that moderate sleep responses to exercise timing to inform more personalized chrono-exercise strategies. REGISTRATION: PROSPERO (CRD42021277708).