Time-of-day variation in endurance and strength performance, and physiological state across the waking day.

The goal of this study was to characterise time-of-day variation in short-duration cycling performance, handgrip strength, and associated physiological and perceptual responses under standardised laboratory conditions. Fifteen healthy recreationally active adults (8 males, 7 females; aged 26.9 ± 2.9 y) completed five experimental trials at 06:00, 10:00, 14:00, 18:00, and 22:00 h in a randomised, counterbalanced repeated-measures design. At each visit, participants completed a 10-min self-paced cycling time trial and assessments of handgrip strength, tympanic temperature, blood glucose, blood lactate, haemoglobin concentration, subjective sleepiness, and perceived exertion. Linear mixed-effects models were used to assess time-of-day effects, and mixed-effects 24-h cosinor analyses were performed as a secondary analysis to estimate peak timing of performance outcomes. Mean cycling power output varied significantly across the day (p < 0.001, ηp2 = 0.507), increasing by 8.4% from 06:00 h to a peak at 18:00 h. Handgrip strength also exhibited significant temporal variation (p < 0.001, ηp2 = 0.495), increasing by 9.0% between 06:00 h and 18:00 h. Cosinor analyses estimated similar acrophases for cycling performance (17:51 h) and handgrip strength (17:27 h). Pre-exercise tympanic temperature (p < 0.001, ηp2 = 0.698), sleepiness (p < 0.001, ηp2 = 0.437), blood glucose (p < 0.001, ηp2 = 0.435), and blood lactate concentrations (p = 0.019, ηp2 = 0.186) varied significantly across the day. Exercise-induced changes in blood glucose (p < 0.001, ηp2 = 0.347) and blood lactate (p < 0.001, ηp2 = 0.295) were also influenced by testing time, whereas post-exercise perceived exertion (p = 0.270, ηp2 = 0.070) and exercise-induced temperature responses (p = 0.140, ηp2 = 0.114) were unaffected. Short-duration cycling performance and handgrip strength are enhanced during the late afternoon and early evening and occur alongside coordinated temporal variation in physiological and perceptual state. These findings highlight time-of-day as a meaningful source of variation in exercise capacity and an important consideration in exercise testing and research design.