Best time of day for strength and endurance training? A systematic review with meta-analysis

Background: Diurnal variations in physiological body functions are known to affect physical performance, but it is unclear if there are time-of-day dependent effects of physical exercise training on performance improvements and health-related physiology.
Objectives: The purpose of this systematic review was to investigate if the time of day at which endurance or strength exercise training is performed affects physical performance, physical fitness, anthropometrics, cardiovascular or metabolic outcomes. To accomplish this, a comprehensive literature search and rigorous assessment of the risk of bias within studies is required.
Study Design: Systematic review with meta-analysis performed in accordance with PRISMA guidelines.
Methods: The databases EMBASE, PubMed, Cochrane Library and SPORTDiscus were searched from inception to May 2021. Eligibility criteria were that the studies (1) included humans, (2) conducted structured exercise training with a minimum of two exercise sessions per week and a minimum training duration of two weeks, (3) compared exercise training between at least two different times of the day, (4) implemented endurance and/or strength training, and (5) used a randomized crossover or parallel group design. There were no restrictions regarding date of publication or participants’ sex, age, fitness level, or health condition.
Results: From 12,783 screened articles, 22 articles were included in the systematic review of which seven were also included in the meta-analyses. The systematic review revealed that there currently is little evidence that training at a specific time of day influences anthropometric, performance-related, and health-related outcome measures more positively compared to other times. Further, the meta-analysis did not provide consistent evidence that training at a specific time of day has a superior impact on performance-related outcomes compared to other times.
Considering the effects of time of day of training relative to time of day of testing, suggests that exercise training in the morning is superior compared to evening training to increase jump performance in the morning -1.11 (95% CI -2.18 to -0.03). However, no significant benefit was observed for such matching of the time of training with the time of testing to improve jump performance in the evening (0.33; -0.12 to 0.77) and to increase strength performance for morning (-0.14; -0.60 to 0.32) or evening (0.34; -0.03 to 0.71). The overall risk of bias in individual studies was moderate to high.
Conclusions: Studies investigating the effects of timing of exercise training on improvements in maximum physical performance so far provide little evidence that there is a time-of-day effect. To date few studies investigated the effects of exercise timing on health-related outcomes. Overall, the risk of bias in these studies is rather high. To provide reliable data on the effects of time of day of exercise on performance-related and health-related outcomes, this systematic review provides recommendations for a rigorous methodological approach for future studies.
Registration: PROSPERO (CRD42021246468)