Effects of Morning Versus Late-Day Exercise on Cardiovascular and Metabolic Outcomes in Different Populations: A Systematic Review and Meta-Analysis

Background: Exercise timing may influence cardiovascular and metabolic responses, but the available evidence varies across populations, exercise protocols, and assessment periods. Objectives: To compare the cardiovascular and metabolic effects of morning and late-day exercise in individuals with and without specified cardiometabolic diagnoses. Methods: PubMed, Web of Science, Embase, Scopus, and the Cochrane Library databases were searched for relevant published literature. We included randomized controlled trials and randomized crossover trials comparing exercise performed within prespecified morning and late-day time windows. Eligible populations included individuals without target cardiometabolic diagnoses and patients with hypertension, diabetes mellitus, coronary artery disease, or metabolic syndrome. Studies were synthesized quantitatively when outcomes were sufficiently comparable for pooling and otherwise summarized narratively. Random-effects models were used for the primary meta-analyses. Results: Thirty-seven studies involving 1150 participants were included. In parallel group trials involving participants without the target diagnoses, late-day exercise was associated with lower blood glucose and triglyceride levels than morning exercise; both estimates were based on two studies and rated as low-certainty evidence. In randomized crossover trials, mean platelet volume was higher after morning exercise both immediately after exercise and during recovery, whereas most other cardiovascular, metabolic, and exercise-response outcomes showed no statistically significant difference between exercise timings. In patients with hypertension and diabetes, pooled analyses did not demonstrate significant differences in the available outcomes, although the evidence was limited and imprecise. Evidence for coronary artery disease and metabolic syndrome was sparse, comprising two and one trials, respectively. Conclusions: Current evidence is insufficient to establish a universally optimal exercise time or demonstrate that differences in acute biomarkers translate into long-term clinical benefit or harm. Although some individual outcomes differed according to exercise timing, the available evidence is limited by the small number of studies, methodological heterogeneity, and limited evidence in populations with cardiometabolic diseases. Larger, well-designed trials with standardized exercise timing protocols and outcome measures are needed to determine whether exercise timing has clinically meaningful effects on cardiovascular and metabolic health.