Sports Medicine - Open · 6 authors, 4 centres
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This systematic review with meta-analysis found little evidence that exercising at a specific time of day (morning vs. evening) leads to greater improvements in physical performance or health outcomes. However, there was some evidence that matching training and testing times (congruency) may enhance performance-related outcomes. The overall risk of bias in the included studies was high, limiting the strength of conclusions.
**Background:** Current physical activity guidelines specify frequency, intensity, type, and duration of exercise but do not address the optimal time of day for training. Diurnal variations in physiological functions (e.g., core temperature, cardiovascular function, endocrine factors) suggest that exercise timing might influence long-term adaptations. This systematic review with meta-analysis aimed to determine whether the time of day of exercise training affects improvements in physical performance or health-related outcomes.
**Methods:** The review followed PRISMA guidelines and was registered on PROSPERO (CRD42021246468). Databases searched included EMBASE, PubMed, Cochrane Library, and SPORTDiscus from inception to January 2023. Eligibility criteria: human studies with structured endurance and/or strength training (≥2 sessions/week for ≥2 weeks) comparing at least two different times of day using randomized crossover or parallel group designs. Two independent reviewers screened 14,125 titles/abstracts, reviewed 55 full texts, and included 26 articles (22 studies) in the qualitative synthesis. Seven studies (191 participants) were included in meta-analyses. Risk of bias was assessed using a modified Cochrane tool. Meta-analyses used random-effects models to compute Cohen's d for changes in maximum strength and jump height, comparing morning vs. evening training and congruent vs. incongruent training-testing times.
**Key Results:** The 22 studies included 713 participants (62% male, 35% female, 3% not reported). Populations included physical education students (8 studies), children (2), healthy elderly (1), adults with type 2 diabetes (3), and other adults (8). Interventions varied: endurance (8 studies), strength (11), combined (3). Mean intervention duration was 9.1 ± 3.7 weeks (range 2–16 weeks). Morning training occurred at ~07:58 ± 36 min, evening training at ~17:19 ± 61 min. Overall risk of bias was high in most studies; only 23% used gold-standard outcome measures, 27% corrected for multiple testing, 9% performed sample size calculations, and 23% were pre-registered.
No consistent time-of-day effect was found for anthropometric outcomes. For performance outcomes, 4 of 10 studies reported significant differences, generally favoring congruency between training and testing times. For health outcomes, 2 of 7 studies found significant differences (evening exercise superior for systolic blood pressure and triglycerides/fasting glucose).
For strength assessed in the morning, the overall effect size (Cohen's d) was -0.07 (95% CI -0.47 to 0.33, I²=0%), favoring morning training but not significant. For strength assessed in the evening, d=0.13 (95% CI -0.27 to 0.53, I²=0%), favoring evening training but not significant. For jump height assessed in the morning, d=-0.73 (95% CI -1.47 to 0.01, I²=80%), significantly favoring morning training (driven by one study). For jump height assessed in the evening, d=0.27 (95% CI -0.17 to 0.71, I²=0%), not significant.
Exploratory meta-analysis comparing congruent vs. incongruent training-testing: For strength, d=0.22 (95% CI -0.15 to 0.59); for jump height, d=0.71 (95% CI 0.00 to 1.42), statistically significant. Comparing morning vs. evening exercise (averaging both test times): strength d=0.13 (95% CI -0.24 to 0.50), favoring evening; jump height d=-0.38 (95% CI -0.83 to 0.07), favoring morning; both non-significant.
**Clinical Implications:** Current evidence does not support recommending a specific time of day for exercise to maximize health or performance benefits. However, for athletes, aligning training time with competition time may enhance performance outcomes. The high risk of bias and heterogeneity across studies limit definitive conclusions. Future research should include diverse populations (e.g., females, evening chronotypes), use gold-standard outcome measures, control for confounders (sleep, diet, chronotype), and pre-register protocols. Until then, the time of day of exercise should not be a primary consideration in exercise prescriptions, but it is not irrelevant and warrants further investigation.