Abstract

Background/Objectives: Repeated sprint performance is typically reduced during the morning because of circadian-related changes in physiological function and muscle strength. Acute caffeine supplementation may attenuate these decrements; however, evidence remains inconsistent. This is partly because of inadequate control of chronobiological and methodological confounders. This study investigated the effects of acute morning caffeine ingestion on repeated sprint performance, physiological responses, strength, power and mood under rigorously standardized chronobiological conditions. Methods: Seventeen recreationally active, low habitual caffeine-consumer males completed six laboratory visits comprising three familiarization sessions and three experimental trials in a performance grouped, randomized for starting condition, counterbalanced, double-blind, placebo-controlled, crossover design. Participants ingested either caffeine (CAFF; 300 mg, 2.5–5.3 mg·kg−1), placebo (PLAC) or no-pill control (NoPill) 60 min before testing (07:30 h). Experimental sessions included assessments of mood, caffeine withdrawal symptoms, intra-aural temperature, grip strength, squat jump, countermovement jump and a 10 × 20 m repeated sprint protocol. Heart rate, ratings of perceived exertion (RPE), thermal comfort, blood lactate and blood glucose were assessed throughout the protocol. Data were analyzed using repeated-measures general linear models. Results: Caffeine did not significantly improve repeated sprint performance compared with PLAC or NoPill, although a trend towards faster 20 m sprint times was observed (3.39 ± 0.19 s vs. 3.43 ± 0.18 s and 3.41 ± 0.19 s, respectively; p = 0.070). Caffeine significantly reduced RPE (p = 0.029) but did not alter heart rate, blood lactate, blood glucose, thermal comfort, intra-aural temperature, mood, sleepiness, caffeine withdrawal symptoms nor lower-body power outcomes. Conclusions: Acute morning ingestion of 300 mg caffeine reduced perceived exertion but did not produce a clear ergogenic effect on repeated sprint performance under rigorously standardized chronobiological conditions. These findings suggest that, in low habitual caffeine users, the central perceptual effects of caffeine are insufficient to overcome the physiological constraints limiting morning repeated sprint performance and highlight the importance of rigorous chronobiological standardization when evaluating nutritional ergogenic aids.