Effects of Hypoxia or Caffeine on 4km Time-trial Morning Cycling.

Time-trial performance is widely regarded as the gold-standard measure of endurance exercise capacity, because successful performance depends on completing a fixed distance in the shortest possible time while effectively regulating pacing strategy and power output. As a result, time-trial performance is considered as one of the most ecologically valid measures of endurance performance in both laboratory investigations and applied sport settings. For many endurance athletes, training and competition frequently occur during the morning due to event scheduling, travel requirements and environmental considerations. However, the morning period is characterised by several time-of-day-related physiological differences that may negatively influence exercise performance. Although systematic reviews have reported substantial variability in the magnitude of diurnal fluctuations in time-trial performance, evidence from well-controlled laboratory studies using fixed-distance protocols under thermoneutral conditions consistently indicates superior performance during the late afternoon or evening. Specifically, improvements in evening performance have been observed across a range of endurance modalities, including 100-m swimming (2.6%), 1-km cycling (2.3%), 4-km cycling (2.6%), 10-km cycling (2.3%), 10-km treadmill running (6.3%), and 16.1-km cycling (3.6%). These findings suggest that morning time-trial performance may be compromised relative to later times-of-day, potentially due to circadian variations in physiological and psychological readiness for exercise. Given that athletes are often required to compete in the morning despite these potential limitations, identifying practical and effective strategies to enhance morning time-trial performance is of considerable interest to athletes, coaches and practitioners seeking to optimise competitive outcomes.

Among the nutritional strategies proposed to improve morning performance, acute caffeine ingestion remains one of the most widely consumed and extensively studied ergogenic aids. Caffeine (1,3,7-trimethylxanthine) primarily exerts its performance-enhancing effects through antagonism of adenosine receptors in the central nervous system, resulting in increased arousal, reduced perceptions of fatigue, and enhanced neurotransmitter activity, including dopamine and noradrenaline. These neurophysiological effects collectively promote wakefulness, attention and central motor drive, thereby facilitating exercise performance. Consistent with these mechanisms, substantial evidence demonstrates that caffeine can improve endurance exercise capacity and time-trial performance across a variety of exercise modalities. Despite the well-established ergogenic effects of caffeine, research examining its efficacy during morning time-trial exercise remains comparatively limited. Interpretation of the available literature is further complicated by several methodological shortcomings, including inconsistent caffeine dosing protocols, inadequate familiarisation procedures and poor control of habitual caffeine intake. Recognising the importance of habitual caffeine consumption as a moderator of physiological and performance responses, recent classification thresholds have been proposed. Categorising individuals as naïve (\<25 mg·day-¹), low (25 mg·day-¹ to 0.99 mg·kg-¹·day-¹), mild (1.00-2.99 mg·kg-¹·day-¹), moderate (3.00-5.99 mg·kg-¹·day-¹), high (6.00-8.99 mg·kg-¹·day-¹), and very high (>9.00 mg·kg-¹·day-¹) caffeine consumers. Failure to adequately characterise and control habitual caffeine intake may therefore contribute to the variability observed across studies.

Additional methodological limitations within the existing literature include the absence of both placebo and no-pill comparator conditions, together with insufficient control of key chronobiological variables such as chronotype, sleep quality, habitual training time, and seasonal influences. These factors are particularly relevant given their established influence on exercise performance and their potential to confound assessments of both circadian variation and caffeine-related ergogenic effects. Indeed, recent systematic reviews have identified substantial methodological heterogeneity and risk of bias across chronobiology studies, highlighting the need for more rigorous and standardised experimental designs when investigating time-of-day effects on exercise performance. Consequently, well-controlled studies are required to determine whether acute caffeine ingestion can meaningfully attenuate the physiological disadvantages associated with morning endurance exercise and improve time-trial performance under ecologically valid conditions.

Therefore, the primary aim of the present study was to determine whether the ergogenic effects of acute caffeine (CAFF) ingestion, administered 60-min before exercise, enhance morning 4-km cycling time-trial performance when evaluated when behavioural, methodological and chronobiological confounders were carefully controlled. Habitual caffeine consumers with an intake of >150 mg·day-¹ were recruited to minimise the potential influence of caffeine tolerance and withdrawal, while maximising the physiological response to the administered 300 mg caffeine dose. Secondary outcomes included measures of core and skin temperature, ratings of perceived exertion (RPE), mood-state and physiological responses to exercise.