Moreover, during repeated sprints or cycling (RSA) tests, performances present the common characteristic to be better in the afternoon (i.e. ~17:00-18:00 h) than in the morning (i.e. ~06:00-07:00 h). Indeed, most of the studies consistently reported higher muscle power (PP) in the afternoon than the morning ^[17–20]; however, this diurnal variation is attenuated when sprints or cycling bouts were repeated ^[17–20], suggesting a possible higher muscle fatigue at the time of the performance peaks ^{[17, 18]}.

However, data examining the effect of time-of-day on long-duration exercise performances appear to be equivocal. In most studies, such diurnal variation is attenuated or seems to dissipate when exercise is prolonged (i.e. for long duration aerobic exercise) ^{[18, 21, 22]}. Indeed, during laboratory testing, although some studies reported that maximal oxygen consumption (VO2_max), ventilator thresholds and/or running/cycling economy fluctuate with the time-of-day ^{[23, 24]}, most studies failed to observe any biorhythmic fluctuations ^[25–29]. However, during field testing, a recent study showed that performances during the Yo-Yo intermittent recovery test (i.e. a progressive shuttle run test) were significantly higher in the evening compared to the morning. Likewise, Souissi et al ^[22] showed that performance during this test was slightly higher (i.e. not-significant difference) in the evening (i.e., 20:00 h) compared to the afternoon (i.e. 14:00 h). This non-significant difference could be due to the selection of the times of testing.

For the repeated cycling exercise, the results of the present study also confirm those of the previous literature ^[17–20].

Consistent with this previous research, an increase of PP during the first two sprints was observed from the morning to the evening. This time-of-day effect could be explained by the diurnal increase in central temperature during sprint cycling^{[19, 20]}. Therefore, it is possible that in the present study, the increase in muscle temperature following the first two sprints “cancelled” out the diurnal increase in central temperature on subsequent sprints. However, to confirm this hypothesis there is a need to measure the central temperature after each sprint of the repeated cycling exercise.

As mentioned above, short-term maximal single or repeated exercises’ rhythms are parallel to the circadian variation in body temperature, peaking in the late afternoon. The increases in body temperature could exert a passive warm-up effect enhancing metabolic reactions, increasing the extensibility of connective tissue, reducing muscle viscosity, and increasing the conduction velocity of action potentials ^{[2, 3]}. Moreover, previous studies reported that EMG activity level was not different between morning and evening testing during discontinuous ^[19] and continuous ^[5] cycling exercises. These studies suggested that the diurnal variations in maximal short-term performance are linked to peripheral, i.e. intramuscular, mechanisms rather than variation in central nervous command. Moreover, Souissi et al ^[12] have suggested that the daily variations of muscle power could be due to a higher aerobic contribution in energy production during the Wingate test in the evening than the morning.

Concerning aerobic exercises, the results of the present study showed a significant time-of-day effect on performances during the Yo-Yo intermittent recovery test. Although, literature's data reported equivocal findings concerning the daytime variations during long duration exercises, the present study confirms those of Hammouda et al.^[18]. The authors showed that TD was significantly higher at 17:00 h than 07:00 h in young soccer players. However, as reviewed by Racinais^[21], the daily variations in maximal oxygen uptake have rarely been observed and studies that have reported significant diurnal variations showed weak amplitudes (i.e. 1-3.8%). To explain the diurnal rhythm of aerobic performances, previous studies have investigated the daily variations in physiological responses to laboratory-based exercise protocols. In this context, Reilly and Brooks ^[27] showed that heart rate and oxygen uptake responses to fixed sub-maximal intensities are higher in the late afternoon. “Likewise, Forsyth and Reilly^[33] showed that the determination of the lactate threshold was affected by time of day in rowing, due to the lowered lactate response to set exercise at night time and in the morning ^[34].