Unlocking the Body's Clock: The Role of Exercise Timing and Chronotype in Cardiometabolic Responses

Kang, J, Ratamess, NA, Faigenbaum, AD, Bush, JA, Fardman, B, Vargas, A, Andriopoulos, T, Melao, J, O'Connell, E, Duff, A, Cmielewski, A, DeSalvo, J, Johnson, A, and Watts, M. Unlocking the body's clock: the role of exercise timing and chronotype in cardiometabolic responses. J Strength Cond Res 39(6): 634-641, 2025-Exercise enhances skeletal muscle metabolism and promotes overall health. However, the time-of-day effect of exercise on physiological responses in humans remains less clear. This study investigated how exercise timing and individual chronotype influence cardiorespiratory and metabolic responses during aerobic exercise. Thirty healthy, young (20.9 ± 1.5 years) adults comprising 15 men and 15 women were recruited and classified using the Morningness-Eveningness Questionnaire into morning types (M-types, n = 5), neither types (N-types, n = 20), and evening types (E-types, n = 5). All subjects underwent a V̇ o2 max test and 2 randomized exercise trials at ∼75% V̇ o2 max for 30 minutes, 1 in the morning before 9 am and 1 in the afternoon after 3 pm . Oxygen uptake (V̇ o2 ), heart rate (HR), respiratory exchange ratio (RER), and rates of carbohydrate (COX) and fat oxidation (FOX) were determined during the last 15 minutes of each exercise bout. No significant differences in V̇ o2 and HR were found between am and pm . However, RER and COX were lower ( p < 0.05) in am (0.920 ± 0.012 and 27.83 ± 2.29 mg·kg -1 ·min -1 , respectively) than pm (0.941 ± 0.09 and 30.65 ± 1.66 mg·kg -1 ·min -1 , respectively), while FOX was marginally higher ( p = 0.077) in am (4.12 ± 0.63 mg·kg -1 ·min -1 ) than pm (3.07 ± 0.49 mg·kg -1 ·min -1 ). In addition, HR was higher ( p < 0.05) in E-types (188.2 ± 3.9) than in N-types (178.4 ± 2.0) and M-types (167.7 ± 4.0). These results suggest that morning aerobic exercise may be more effective in mobilizing fat as an energy source compared with afternoon exercise. The similar V̇ o2 and HR levels between morning and afternoon sessions imply that the time of day may not have a major impact on cardiorespiratory responses during submaximal aerobic exercise. However, exercising HR seems to vary across chronotypes, highlighting the need to consider individual circadian phenotypes when conducting training programs.