Abstract
Purpose Time-of-day variation in exercise performance is well established, but the contribution of circadian phenotype to the magnitude of these differences remains unclear. This study examined diurnal variation in aerobic exercise performance and determined whether chronotype and sleep timing were associated with morning–evening differences in performance. Methods Twenty-four healthy young adults (12 male, 12 female) completed a randomised crossover protocol consisting of a 10-min cycling time trial performed in the morning (08:00–09:00) and evening (17:00–18:00). Pre-trial sleep, diet, and physical activity were standardised, and circadian phenotype was characterised using the Composite Morningness Questionnaire and 7-day actigraphy. Results Mean power, peak power, and distance covered were all significantly higher in the evening compared with morning ( p < 0.01). Chronotype ( r = − 0.47, p = 0.019) and sleep midpoint ( r = 0.52, p = 0.003) were significantly associated with the magnitude of the diurnal performance difference, accounting for 22% and 27% of the variance, respectively. Heart rate, blood lactate, and perceived exertion responses were similar between conditions, indicating comparable physiological strain. Core temperature was higher in the evening ( p = 0.026), but did not explain performance differences. Conclusions These findings demonstrate that endurance exercise performance is enhanced in the evening compared with the morning and suggest that behavioural markers of circadian phenotype are associated with the magnitude of diurnal variation in performance. Time-of-day and circadian phenotype should therefore be considered when optimising exercise testing and prescription.