Time-of-Day Effects of Exercise on Cardiorespiratory Responses and Endurance Performance—A Systematic Review and Meta-Analysis

Abstract Kang, J, Ratamess, NA, Faigenbaum, AD, Bush, JA, Finnerty, C, DiFiore, M, Garcia, A, and Beller, N. Time-of-day effects of exercise on cardiorespiratory responses and endurance performance—A systematic review and meta-analysis. J Strength Cond Res 37(10): 2080–2090, 2023—The time-of-day effect of exercise on human function remains largely equivocal. Hence, this study aimed to further analyze the existing evidence concerning diurnal variations in cardiorespiratory responses and endurance performance using a meta-analytic approach. Literature search was conducted through databases, including PubMed, CINAHL, and Google Scholar. Article selection was made based on inclusion criteria concerning subjects' characteristics, exercise protocols, times of testing, and targeted dependent variables. Results on oxygen uptake (V̇ o 2 ), heart rate (HR), respiratory exchange ratio, and endurance performance in the morning (AM) and late afternoon or evening (PM) were extracted from the chosen studies. Meta-analysis was conducted with the random-effects model. Thirty-one original research studies that met the inclusion criteria were selected. Meta-analysis revealed higher resting V̇ o 2 (Hedges' g = −0.574; p = 0.040) and resting HR (Hedges' g = −1.058; p = 0.002) in PM than in AM. During exercise, although V̇ o 2 remained indifferent between AM and PM, HR was higher in PM at submaximal (Hedges' g = −0.199; p = 0.046) and maximal (Hedges' g = −0.298; p = 0.001) levels. Endurance performance as measured by time-to-exhaustion or the total work accomplished was higher in PM than in AM (Hedges' g = −0.654; p = 0.001). Diurnal variations in V̇ o 2 appear less detectable during aerobic exercise. The finding that exercising HR and endurance performance were greater in PM than in AM emphasizes the need to consider the effect of circadian rhythm when evaluating athletic performance or using HR as a criterion to assess fitness or monitor training.