This study was designed to assess the effects of time-of-day on aerobic and anaerobic performances during the Yo-Yo, repeated sprint ability (RSA) and the Wingate tests in young soccer players. The major finding of the present study is that aerobic and anaerobic performances of trained young soccer players fluctuate with the time-of-day. Indeed, Ppeak
during the Wingate test, PP (i.e. during the first two sprints), Pdec and Wtotal during the RSA test; and TD and MAV during the Yo-Yo test were significantly higher at 17:00 than 07:00 h.Concerning anaerobic performances (i.e. the Wingate test), the present study's results confirm those of previous studies in adult active subjects [1–3, 5, 9–15]
or soccer players [4, 6, 17, 18]
. Likewise, the diurnal variation of short-term maximal performances was observed in boys [7, 8]
. During the Wingate test, it has been showed that Ppeak
powers fluctuate with time-of-day (i.e. circadian rhythmicity), with morning nadirs, afternoon/early evening highest values and a peak-to-trough amplitude equal to 7.6±0.8% and 11.3±1.1%, respectively 
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 
and continuous 
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 
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.
. 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
, 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 
showed that heart rate and oxygen uptake responses to fixed sub-maximal intensities are higher in the late afternoon. “Likewise, Forsyth and Reilly
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 
This study suffers from some limitations such as the inclusion of a control group to compare the amplitude of the rhythm between the footballers and the untrained subjects. Moreover, the present study could be completed by other investigations using more specific field testing with taking into account the environmental conditions (i.e. as a previous study showed environmental changes affect the diurnal rhythm of sports performance