Human performances are highly dependent on one of the main biological rhythms, namely the sleep–wake rhythm. This rhythm is driven by the living clock and is functional in the adaptation to day–night differences in the environment. The athlete, like all humans, is set up to be awake and in good shape to exercise during the day, and to sleep and recover during the night. Regular vigorous physical activity, such as the light–dark cycle or social cues, can be considered as a zeitgeber. It has been shown to reinforce the synchronisation of human circadian rhythms, which means that physically active people have a stronger circadian rhythmicity than those who are sedentary. Sleep and exercise also have strong relationships that are independent of the biological clock. On the one hand, athletic performances have been shown to be dependent upon both the quality and the quantity of sleep that has been taken before the competition. The detrimental effects of sleep deprivation are shown by increased lapsing, cognitive slowing, memory impairment, decreased vigilance and sustained attention, and shift in optimum response capability. Its effects on physical performance are manifested as a decline in the ability to perform maximal exercise. Aerobic and anaerobic pathways are both affected, as are fatigue and recovery processes. Sleep deprivation can also phase shift and decrease the amplitude of many individual rhythms. All these effects are dependent on the timing and the length of the waking state, but many of them are seen after only few hours of sleep deprivation. On the other hand, exercise by itself has been shown to have an impact on subsequent sleep. Sleep latencies and fragmentation are reduced and the deep slow wave sleep is increased. It is beyond doubt that athletes need more sleep than sedentary people, and so athletes, in order to perform optimally, need to respect the “rules” imposed by the circadian pacemaker for establishing consistent sleep periods.

The aim of this study was to evaluate time-of-day effects on fatigue during a sustained anaerobic cycling exercise. Sixteen healthy male competitive cyclists were asked to perform a 60 s Wingate test against a braking load of 0.087 kg.kg body mass⁻¹ during two experimental sessions, which were set up either at 06:00 or 18:00 h in counterbalanced order. There was only one session per day with a recovery period of at least 36 h between the two sessions. Each subject was trained to perform the test. The body mass used to determine the braking load was that of the first test session for each subject and remained constant throughout the two test periods. During the test, peak power (PP), mean power during the first 30 s (MP30 s) and the full 60 s of the test (MP60 s), and fatigue (i.e., the decrease in power output values throughout the exercise) were analyzed. Results confirmed the existence of diurnal variation in anaerobic power output. PP, MP30 s, and MP60 s were significantly higher at 18:00 than 06:00 h, with gains equal to 8.2, 7.8, and 7.8%, respectively. Moreover, all the power output values recorded in the evening were higher than those recorded in the morning, indicating that fatigue induced by this exercise is not affected by time-of-day in male competitive cyclists. It is hypothesized that the freedom and complexity of pedalling could allow adaptations in movement patterns, as a function of time-of-day, in order to maintain higher performance in the evening. For practical considerations, the more complex the movements required to perform a sport, the more the time-of-day effect can be taken into account and adapted to by the trained athlete, particularly in cyclic sporting disciplines such as swimming, running, rowing, and kayaking. (Author correspondence: damien. davenne@unicaen. fr )