Various psychological and physiological functions have been shown to undergo changes relative to the time of the solar day (4). These variations are known as circadian or diurnal rhythms (13). In this context, it has been well documented that maximal short-term performances fluctuate with time of day, with morning nadirs and afternoon maximum values (16–18,108–111). These diurnal or circadian variations in high-intensity short-duration exercises (e.g., muscle power, muscle strength, sprint) have been found to range from 3 to 21.2%, depending on the population tested, the muscle groups and the experimental design (78). However, the literature regarding circadian or time-of-day effects on aerobic exercise (e.g., maximal oxygen consumption [V[Combining Dot Above]O2max], maximal aerobic velocity, time to exhaustion) yields inconclusive results. For example, although some studies reported a significant diurnal variation of aerobic performance (46,48,49,56,119), other investigations failed to confirm any biorhythmic variation (24,25,98,115). One reason for these conflicting results may be that subjects not satisfying the criteria that V[Combining Dot Above]O2max is actually attained (at any time of day) (27).
The diurnal variations in sport performances can be influenced by several factors such as sleep deprivation, warm-up duration, and training time of day (16,18,111). Based on previous literature, training in the morning can (a) improve typically poor morning performances to the same or even higher level as their normal daily peak typically observed in the evening and (b) decrease the amplitude of the diurnal variations (16,18,111). However, training in the evening hours can increase the daily variations' amplitude of neuromuscular performances (16,18,111).
In view of the above considerations, we aim to (A) review (a) the effect of time of day on long and short-duration exercise performances and the possible explanation of such rhythmicity and (b) the effect of training at a specific time of day on the diurnal rhythm of performances and (B) appraise some mechanisms that could be responsible for such temporal specificity of training.
Diurnal Variations of Sport Performances
The human body is dependent on biological rhythms that affect both mental and physical activities (4,20). The effects of time of day on short-term anaerobic performances are well documented either when tests are conducted over the entire solar cycle 24 hours (generally use the terminology of “circadian rhythm”) (110) or when they are conducted only during the daytime (generally use the terminology of “diurnal rhythm”) in adults (16–19,21) or youth subjects (105,106). Indeed, the effect of the circadian or diurnal variations has been demonstrated for various indices of short-term maximal performances during continuous (e.g., Wingate test) (16–19,21,108–111) and intermittent exercises (e.g., repeated-sprint ability test) (45,46,83,88) or very brief all-out efforts (e.g., maximal voluntary contraction (MVC), squat jump, countermovement jump, etc.) (16,18,35,70,103) (Table 1). These short-duration performances present a common characteristic to be better in the afternoon (e.g., the diurnal maximum is almost always found between 16:00 and 20:00 hours) than in the morning (e.g., the diurnal minimum is almost always found between 06:00 and 10:00 hours). These daily variations have been found to range from 3 to 21.2%, depending on the population tested, the muscle groups and the experimental design (78).
For example, during the Wingate test, it has been well documented that peak and mean powers fluctuate with time of day, with morning nadirs, afternoon and early evening highest values (16–19,21,108–111), and a peak-to-trough amplitude equal to 7.6 ± 0.8 and 11.3 ± 1.1%, respectively (Figure 1) (110).
Possible Explanations of the Diurnal Rhythm in Short-Duration Exercises
Although neuromuscular performances have been repeatedly shown to be lower in the morning compared with the afternoon and evening, the scientific available data provide somewhat conflicting results on the origin and mechanisms of such diurnal rhythms.
As mentioned above, there is fairly comprehensive evidence of circadian rhythms in many aspects of human performance, including athletic performance (4). Many of these rhythms are parallel to the circadian variation in body temperature, peaking in the late afternoon (Figure 1) (110). Based on this finding, some earlier studies have suggested that the simultaneous increases in core temperature and short-term maximal efforts are causally related and that the circadian rhythm of core temperature could exert a passive warm-up effect (6,72) enhancing metabolic reactions, increasing the extensibility of connective tissue, reducing muscle viscosity, and increasing the conduction velocity of action potentials. In this context, Bergh and Ekblom (5) demonstrated that power output decreased by 5% for every 1° C decline in muscle temperature in warming and cooling experiments for muscle temperatures between 30 and 39° C. Recent studies (83,86) suggest that exposure to a warm (>28° C) and humid environment can increase short-term maximal performances only in the morning, when body temperature is at its lowest, and, therefore, reduces the amplitude of their diurnal rhythm. The authors concluded that both heat exposure and diurnal increase in body temperature can cause a passive warm-up effect improving muscle contractility, but these 2 factors cannot be combined to improve short-term maximal performances (84).
However, other authors suggest that the diurnal difference in core temperature was not the only explanation of the time-of-day effects on short-term anaerobic performances (21,70,109). Martin et al. (70) showed that the diurnal fluctuation in muscle strength persisted despite an artificial heating of the adductor pollicis muscle by 5° C at 07:00 hours. Therefore, as observed by Martin et al. (70), although core temperature and short-term maximal performances do covary, causality is not necessary concluded. It is likely that they are powered by a common central oscillator (36).
To summarize, the circadian rhythm of body temperature may partially but not solely explain the diurnal variations in short-term maximal performances (e.g., muscle strength and power), and the majority of authors have proposed its multifactorial origins.
In recent studies, surface electromyography (EMG) has been widely used to discriminate the involvement of peripheral (contractile state of the muscle) and central (neural input to the muscles) mechanisms in the diurnal variations of short-term maximal performances (21,76,78,83,88,101). However, again, the scientific literature yielded inconclusive results. In fact, Gauthier et al. (35) and Castaingts et al. (15) reported that both central and peripheral mechanisms may be altered across the day. For example, Gauthier et al. (35) showed that the muscle activity (i.e., EMG activity) produced by the arm flexors was higher in the afternoon than in the morning. Likewise, Callard et al. (12) found higher EMG activities in the evening and attributed part of the diurnal variation in MVC to changes in the central nervous command. However, other studies have suggested that the diurnal variations in maximal short-term performance are linked to peripheral, that is, intramuscular, mechanisms rather than variation in central nervous command (21,39,70,76,78,83,88,103). Indeed, previous studies reported that EMG activity level was not different between morning and evening testing during isokinetic (39,76), discontinuous cycling (88), and continuous cycling (21) exercises. To confirm these findings, previous studies have used the twitch interpolation technique to assess the level of muscle activation or the central activation capacity, which refers to the level at which the motoneuron pool has been voluntarily excited to produce tension in the muscle of interest. Indeed, better muscle contractile properties in the afternoon than in the morning have been evidenced by a higher mechanical response to an electrical stimulation of the motor nerve (i.e., maximal twitch tension) (70) and a higher root mean square/force ratio during voluntary contraction in the afternoon than in the morning (21,83).
In addition to the diurnal variations in EMG activities and core temperature, previous studies reported changes in both the architecture (i.e., the arrangement of muscle fibers relative to the axis of force generation) of vastus lateralis muscle fibers (80,81) and patella tendon stiffness (81) over the course of the day (i.e., higher in the morning). These findings suggest that the subjects were bound to have used their muscles in the evening more than in the morning test, hence possibly causing the tendon to become more compliant. In addition, Onambele-Pearson and Pearson (79) showed that patellar tendon stiffness was 20% higher in the morning compared with that in the afternoon. However, surprisingly, this had no detrimental effect on the force produced because there was 16% increase in MVC torque from the morning to the afternoon. Recently, Lericollais et al. (67) showed that the diurnal variations in muscle power during a 60-second Wingate test (i.e., higher values in the evening) were associated with changes in cycling kinematic parameters, characterized by a reduction in the range of motion of the ankle angle in the evening during the Wingate test. Moreover, Souissi et al. (108) have suggested that the daily variations of muscle power during the Wingate test is mainly because of a higher aerobic contribution in energy production, faster V[Combining Dot Above]O2 kinetics, and better net efficiency (work performed/energy expended above that at rest) (11) in the evening than in the morning.
Biorhythmicity of Physiological Parameters and Long-Duration Exercises
Although diurnal rhythms in short-duration exercise performances have been widely described, in most studies, such variations seem to dissipate when exercise is prolonged (89). Indeed, data reporting the effect of time of day on long-duration exercises appear to be equivocal (Table 2). In this context, previous investigations failed to show circadian or diurnal variations in time to exhaustion during maximal cycling exercise (25,93), total work and average power output during a 15-minute time trial cycling (24) and work rate during submaximal cycling (28). Likewise, Souissi et al. (107) showed that the total distance covered during the Yo-Yo intermittent recovery test does not fluctuate with the time of day. In contrast, Hammouda et al. (46) demonstrated that the distance covered during the Yo-Yo test was higher in the evening compared to the morning. However, a limitation of this field test is that the exercise is subject to voluntary exhaustion and so the participant must be highly motivated to achieve the end-point on each occasion the test is conducted. Another investigation demonstrated that tolerance of high-intensity exercise on a cycle ergometer is time of day dependent on morning performances inferior to the evening ones (100). Bessot et al. (8) showed that time to exhaustion at 95% of maximal power during a cycling exercise is higher at 18:00 than at 06:00 hours. Moreover, Atkinson et al. (3) observed that performances during the 16.1-km cycling time trial exercise are worse in the morning than in the evening.
The convergent results concerning the effect of time of day on long-duration exercises performance could be because of the reluctance of some subjects to exercise to voluntary exhaustion during troughs in arousal. Indeed, in such instances the level of performance (e.g., time to exhaustion, submaximal cycling time, total distance covered during the Yo-Yo test) depends on the voluntary effort provided by the subject (98).
Moreover, time-of-day affects performance in a range of sports, including time trials in cycling, rowing, swimming, shot put (23), and skills in badminton, football, and tennis (see Reilly and Waterhouse ).
Resting metabolic functions, also, display circadian rhythmicity (Table 3). Indeed, the rhythms in V[Combining Dot Above]O2 and in carbon dioxide production (V[Combining Dot Above]CO2) have an amplitude of about 7% (91). This compares with a rhythm in minute ventilation (VE) of about 11% (91). This indicates that the respiratory exchange ratio (RER; V[Combining Dot Above]CO2/V[Combining Dot Above]O2) does not display circadian rhythmicity.
The circadian rhythm in resting physiological variables would be maintained during exercise. However, some rhythms disappear, whereas others become more marked during exercise (i.e., higher amplitude).
Middle- and long-duration performances are partly linked to the athlete's V[Combining Dot Above]O2max, maximal heart rate (HRmax), ventilatory thresholds, and running-cycling economy. However, again, there is no clear evidence that these physiological responses to exercise fluctuate during the day (89) (Table 4). The daily variations in V[Combining Dot Above]O2max have rarely been observed (49,50,119), and most investigations failed to observe any biorhythmic variation in V[Combining Dot Above]O2max (24,25,91,98,115). In addition, studies that have reported significant diurnal variations in V[Combining Dot Above]O2max showed weak amplitudes of between 1 and 3.8% (49,119).
From the current literature's findings, it seems that the effect of time of day on physiological responses to long-duration exercises weakens (i.e., the amplitude of the rhythm becomes relatively smaller) or disappears as the exercise intensity is increased toward maximum. Indeed, the circadian variation, evident at rest (amplitude 5% of mean), becomes indiscernible at V[Combining Dot Above]O2max where the amplitude of the rhythm at rest would represent only about 0.2% of the maximal value (93).
The V[Combining Dot Above]O2max itself does not seem to exhibit circadian or diurnal variation, even though some research reports (49,50,119) have indicated the opposite. There are standard criteria for indicating whether V[Combining Dot Above]O2max has been attained (e.g., a plateau in V[Combining Dot Above]O2 despite increase in power output; RER >1.10; heart rate equal to predicted maximal values). In most studies, the subjects failed to demonstrate 2 out of the 3 criteria that should be recalled for a repeated test on another occasion (27,91). Therefore, significant circadian rhythms may not be evident in all exercises of maximal physiological capacities, when measurement error approaches the range of the rhythm amplitude.
Endurance exercise does not typically engage the V[Combining Dot Above]O2max because it is performed at a submaximal intensity (i.e., expressed as a percentage of V[Combining Dot Above]O2max) (93). The influence of the circadian rhythm on core temperature, V[Combining Dot Above]O2, V[Combining Dot Above]CO2, VE, and HR has been shown at rest (93) and during submaximal exercises (93,96), indicating that rhythms apparent at rest can persist into exercise. Indeed, the circadian rhythm in temperature persists during submaximal exercise, as does that of HR, V[Combining Dot Above]O2 (38,98), and VE (98). In this context, a daily variation has been observed previously for V[Combining Dot Above]O2 kinetics during moderate intensity cycling exercise (11), but not during high-intensity running (14) or cycling (8) exercise. Moreover, Forsyth and Reilly (32) reported that lactate threshold was affected by time of day in rowing, because of the lowered lactate response to set exercise at night and in the morning. Dalton et al. (24) showed that higher blood lactate levels at rest and during exercise occurred between 14.00 and 16.00 hours, following a pattern similar to that of body temperature. During and after maximal intensity exercise, Deschenes et al. (25) found that blood lactate concentrations were higher in the evening. However, in this study, subjects exercised longer in the evening compared to the morning, which may be responsible for the elevated lactate levels later in the day.
Concerning the effect of time of day on rating of perceived exertion (RPE), a diurnal variation has been previously shown (49). Indeed, Hill et al. (50) showed a significant diurnal variation in RPE during a long-duration exercise above the ventilatory threshold (VT) with higher scores in the morning compared with the evening; however, no time-of-day effect was observed for intensities below the VT. The authors have attributed this difference to the generally lower morning ventilatory demand. However, Faria and Drummond (29) and Ilmarinen et al. (51) showed a diurnal variation in RPE during maximal exercise, with the maximal ratings occurring in the afternoon or evening. However, more recent research studies, did not show significant diurnal variations on RPE after submaximal (25,28,73) and maximal cycling exercise (25) and time trial performance (24).
Circadian rhythm of body temperature is among the most prominent rhythms associated with physical performance. Indeed, circadian rhythmicity has been reported for muscle performance measures, with a cycle closely corresponding to the body temperature curve (97). In this context, Hill et al. (49) showed significant correlations between the afternoon increased body temperature and the V[Combining Dot Above]O2/work rate slope above VT and between the afternoon increased body temperature and V[Combining Dot Above]O2 below the VT during a maximal graded cycle ergometer. The authors concluded that the higher body temperatures increase the demand on the respiratory and thermoregulatory systems at a given exercise intensity resulting in a higher V[Combining Dot Above]O2 in the evening. Body temperature could also affect the diurnal variations of metabolic variables. Indeed, using the Q10 (i.e., measure of the rate of change in a biological or chemical system after increasing temperature by 10° C) effect of temperature, the contribution of the stored heat is about 33% of the observed circadian rhythm in metabolism (97). In this context, Reilly and Brooks (96,97) showed that maximal HR and core temperatures in response to an incremental exercise (20–25 minutes) are reached at the time of day when the body temperature is at its higher value, that is, in the evening, with an amplitude of 0.9° C, which would account for 62, 68, and 72% of the change observed in VE, V[Combining Dot Above]O2, and HR, respectively. The circadian rhythm of body temperature has been found to persist in any exercise intensity (see Reilly and Waterhouse ). However, the relationship between performance during long-duration exercises and core body temperature may be disrupted when exercise causes a pronounced rise in body temperature. Indeed, Reilly and Garrett (98,99) showed that athletes started at higher exercise intensity in the evening compared with the morning, but this superiority was reversed after 90 minutes of exercise. The changes were attributed to approaching hyperthermic fatigue, the core temperature being about 0.6° C higher at the morning start than the evening one (93).
The circadian variation in blood pressure (BP) has been confirmed with the lowest BP during sleeping (i.e., nocturnal dip equal to 10–20% of the daytime mean level), a morning “surge” around the waking hours, and the highest BP at midday (53–55,96). The early surge in BP could be because of an endogenous circadian rhythm, for example, arousal effects of awakening, an activation of the sympathetic nervous system, and other hemodynamic adjustments that occur after arising from bed or a combination of all these influences (53–55). Exercise training is advocated as a nonpharmacological intervention to lower BP (53). Indeed, an acute bout of exercise causes a reduction in BP compared with the preexercise level, which is termed “postexercise hypotension.” However, after early morning exercises, that is, specifically between 04:00 and 08:00 hours, this phenomenon is absent or even reversed (53). The authors concluded that, for greatest acute BP-lowering effects, exercise should be scheduled in the afternoon. At this time of day, exercise induces a specific heat load that results in smaller temperature increases and larger cutaneous vasodilatory responses (118), suggesting a potential for greater BP reduction that appears to enhance thermoregulatory responses in the afternoon compared with the morning. From a clinical perspective, Jones et al. (53–55) concluded that the higher postexercise BP values found in the morning coincide in time with higher incidence of cardiac death and stroke. Indeed, myocardial infarction, sudden cardiac death and stroke incidence show circadian peaks between 06:00 and 12:00 hours (58). Moreover, the morning surge in BP has been implicated in disturbing vulnerable plaques, which in turn can cause an acute cardiac event (75). Comparing intermittent and continuous exercise, Jones et al. (55) reported that intermittent exercise mediates greater postexercise hypotension compared with a single continuous bout of equivalent work and that this protocol-dependent difference is greatest in the afternoon. The authors concluded that a bout of afternoon exercise that is occasionally interrupted with short rest periods is recommended for lowering BP acutely.
Training at a Specific Time of Day
Studies regarding the effect of aerobic training scheduled in the morning or in the evening showed that training' adaptations are time of day dependent. Hill et al. (48) have suggested that there is a time-of-day specificity in an aerobic training designed to improve the anaerobic threshold (i.e., the highest sustained intensity of exercise for which measurement of oxygen uptake can account for the entire energy requirement). The authors showed that, after 6 weeks of training, the ventilator anaerobic threshold was higher in the morning than in the evening for the morning training group (i.e., subjects who had trained in the morning), and it was higher in the evening for the evening training group (i.e., subjects who trained in the evening). However, the control group threshold (i.e., subjects who did not train) was the same in the morning and the evening both before and after the experimental period. In another study, Torii et al. (115) found that 4 weeks of endurance training enhanced V[Combining Dot Above]O2max only when testing was conducted at the same time of day as training had been scheduled. The participants had trained at 3 times (i.e., in the morning, afternoon or evening). However, V[Combining Dot Above]O2max testing was performed only in the afternoon both before and after training for the 3 groups. In addition, the authors suggested that adaptations to training were greater when aerobic training was performed in the afternoon than at other times. Likewise, Hill et al. (50) showed a temporal specificity after 5 weeks of high-intensity training designed to increase work capacity. The evening training group had greater work capacity at this time of day. However, performances of the morning training group were not time of day dependent. The authors suggested that a greater improvement appeared to occur at the same time of day at which high-intensity training is regularly performed. However, because both the morning and the evening training groups were not evaluated before training, the authors could not compare the improvement from before to after the regular training at a particular time of day (i.e., if the improvement of performance was greater at the same time of day of regular training).
Surprisingly, the question of whether resistance training adaptations may depend on the time of day of training has received little scientific attention up to date (Table 5). The assumption that adaptations to resistance training may differ from the regular time of day of training is based on the fact that various limiting factors of physical performance are highest at certain time of day. Souissi et al. (111) showed significant time-of-day specific adaptations after 6 weeks of resistance training designed to promote muscle strength and power. Indeed, the subjects who trained in the morning hours improved their performances (i.e., muscle power during the Wingate test) in the morning and in the evening. However, those who trained in the evening hours improved their performances only at this time of day. The authors suggest that adaptations to strength training are greater at the time of day at which training was conducted than at other times. Likewise, we recently showed that performance (i.e., muscle strength and power) improvements after 6 weeks of resistance training are greater at the time of day at which training was scheduled than at other times (18). In fact, performances during the 1-repetition maximum (1-RM), the squat jump, the countermovement jump and the Wingate tests was greater in the morning in subjects who trained at 07:00 hours and in the evening in subjects who trained at 17:00 hours. In addition, Souissi et al. (111) and Chtourou et al. (18) showed that training at a specific time of day could modify the typical diurnal pattern of short-term maximal performances. Indeed, the diurnal variations in anaerobic performances were blunted in the morning training group. However, the amplitude of the intradaily variations of short-term maximal performances increased in the evening training group, which is the time of day when performance typically is most impressive. Likewise, Sedliak et al. (102–104) showed that the typical diurnal pattern of maximum isometric strength was blunted after 10 weeks of resistance training in the morning but not the evening group. The same results are observed by Souissi et al. (106) after 6 weeks of resistance training in youth subjects.
Furthermore, Blonc et al. (10) showed that 5 weeks of various exercises training (repeated sprints, jumps, standard track and field exercises; training duration = 50 minutes with 10-minute warm-up) designed to develop muscle power can improve performances during the squat jump and countermovement jump tests for both the morning and the evening training groups with the same benefits. Indeed, they failed to show any time-of-day specific adaptations on either performances or training benefit. The authors suggested that the passive warm-up effect of the environment might be the cause of the no effect of the training at a specific hour. In fact, this study was conducted in Guadeloupe, French West Indies, which has a warm and moderately humid natural environment (mean environmental temperature and humidity were 27.9 ± 0.5° C and 65.4 ± 12% outside). The passive warm-up effect of this environment has been suggested to blunt the passive warm-up effect of time of day (85) and, thus, may lead to specific physiologic adaptations to exercise and certainly influences the circadian regulation of some neurohormonal metabolisms (10).
In summary and for practical consideration, if the time of competition is known, training sessions before a major competition must be conducted at the same time of day at which one's critical performance is programmed. However, if the time of competition is not known, resistance training sessions should be programmed in the morning to improve short-term maximal performances and counteract the effect of time of day (i.e., the morning nadir of performances). However, all these studies (18,102–104,106,111) have not investigated changes in performance during the course of the day in high-level athletes; this is important because the amplitude of the diurnal rhythm is higher in active compared with inactive subjects (2).
Tapering at a Specific Time of Day
The taper is a training phase of progressive reduction in the training load for a variable period of time before a major competition to allow for physiological and psychological recovery from accumulated training stress. This reduction of the training load can be achieved by the alteration of several training' components such as the training volume, intensity, and frequency, and the pattern of the taper (i.e., progressive or step taper) and its duration (74). The rationale behind this training phase is the advantage of reduced fatigue levels to enhance training tolerance and respond effectively to the training undertaken during the taper. Unfortunately, only one study has investigated the effect of tapering in the morning or in the evening hours on subsequent performances and their diurnal rhythms.
Regarding this topic, although the effect of tapering (itself) on performance cannot be isolated, we showed that 12 weeks of resistance training and 2 weeks of tapering (i.e., reduction of 50% of the weekly training volume) at the same time of day may alter the normal diurnal variations of muscle strength and power (16). Indeed, adaptation to resistance training and tapering during this study was greater at the time of day at which training is regularly performed than at other times. Moreover, resistance training and tapering performed at 07:00 hours may improve typically poor morning performances to the same or even higher level as their normal daily peaks typically observed in the evening (16). However, although the relative increase in performances was slightly higher after evening tapering, we showed that the increase in short-term performances after a tapering phase was unaffected by the time of day of training (16).
Given the lack of well-controlled investigations that have examined the optimal time of day of training and tapering for athletes, future studies are required to determine the optimal time of day of tapering. Tapering and reduced training schemes as function of the time of day of training could then be developed to avoid physical overstress and prevent detraining
To better understand the potential mechanisms involved in training and tapering at a specific time of day, the following section will review the main physiological changes accompanying training adaptations.
Possible Explanations of the Time-of-Day Specific Adaptations
Hormonal Adaptations with Special Reference to Testosterone and Cortisol
The acute hormonal responses to resistance training are very important because anabolic hormones such as Testosterone (T) (30) and growth hormone (33) will increase protein synthesis in muscle cells (i.e., increase of contractile proteins) (41–43). The correlation reported by Ahtiainen et al. (1) between T and changes in isometric strength and cross-sectional area (CSA) suggests that T may be an important signaling factor for strength and hypertrophy development. One of the main functions of T is to maintain anabolism of the body by promoting protein synthesis within the muscular system (30). Indirect evidence implies that muscle strength and hypertrophy development may be positively dependent on resting levels of total and free T levels (40). Moreover, resistance training may increase androgen receptor expression in muscle cells (117) and, therefore, provide more binding sites that may possibly result in higher physiological activity of T (57,64,65,69,71).
However, considered to be a catabolic hormone, Cortisol (C) is often referred to as a ‘‘stress’’ hormone, and previous studies have documented that prolonged elevation of C levels have been associated with a higher risk of muscular atrophy and strength deficits (82). Therefore, because C primarily affects protein degradation, a decrease in C is expected to enhance skeletal muscle hypertrophy through reduction in protein degradation rather than increase in protein synthesis as the primary mechanism (47).
Both T and C display a circadian pattern with peak concentrations in the morning, around the commencement of diurnal activity, and reduced concentrations in the evening and overnight (116). The morning rise in C accelerates metabolism (31) and stimulates gluconeogenesis and proteolytic activity, resulting in increased skeletal protein turnover (26). The increase in T at this time may be an attempt to counteract the stimulatory effect of C on skeletal protein degradation (60).
Regarding the effect of a resistance training session performed at different time of day, there appears to be only one study that has raised this question and demonstrated that the acute resistance exercise (RE) induced hormonal responses can be influenced by the circadian time structure (9). The authors observed that RE performed in the evening positively altered the C (higher decrease in the evening) and then T/C ratio compared with the morning which resulted in a reduced catabolic and increased anabolic environments which therefore could optimizes skeletal muscle hypertrophic adaptations associated with resistance training.
Regarding the effect of regular resistance training at a specific time of day, unfortunately, also only the study of Sedliak et al. (104) has raised this question. However, Sedliak et al. (104) showed that only C concentrations decreased significantly in subjects who regularly trained in the morning hours, while training in the morning or evening hours had no effect on resting serum T concentrations. However, the authors suggested that this reduction in serum C may presumably be because of a decreased anticipatory psychological stress before the morning test sessions rather than to adaptations induced by a regular training at this time of day. Moreover, for muscle hypertrophy, Sedliak et al. (102) observed that the magnitude of the development of the quadriceps femoris CSAs and volume did not differ after regular training in the morning or in the evening hours.
Hormonal Responses With Special Reference to Testosterone and Cortisol
Concerning the effect of acute RE scheduled at different time of day (i.e., morning or evening training session) on hormonal responses, Bird and Tarpenning (9) observed that RE performed in the evening compared with the morning positively altered the C (higher decrease in the evening) which resulted in a reduced catabolic and increased anabolic environment, therefore optimizing skeletal muscle hypertrophic adaptations associated with RE at this time of day. In the same context, Kraemer et al. (62) investigated the effect of a resistance training session (i.e., 10 repetitions × 3 sets in 10 exercises with 2 min recovery in between) on the circadian rhythm of the salivary testosterone concentrations in trained men. However, the authors showed that RE does not affect the circadian pattern of salivary T. Likewise, Deschenes et al. (25) investigated the effect of RE protocols on an isokinetic dynamometer with maximal effort performed at different time of day (i.e., 0800, 1200, 1600, and 2000 hours) on plasma testosterone and cortisol concentrations. The authors showed that this resistance training protocol has no effect on the circadian rhythm of testosterone and cortisol concentrations. Indeed, they reported that testosterone and cortisol displayed significant biorhythmicity both before and after the exercise.
As mentioned above, previous studies suggest that a typical diurnal pattern of short-term maximal performances might be altered by training at a particular time of day. Another possible explanation for these specific adaptations may stem from neuromuscular adaptations to resistance training. However, only one study has investigated the roles of central and peripheral mechanisms involved in the temporal specificity of resistance training (103). The authors failed to show any adaptation to resistance training scheduled repeatedly at a particular time of day, on the electromyographic activity of the knee extensors during unilateral isometric knee extension peak torque. Thus, they suggested that peripheral rather than neural adaptations are the main source of temporal specificity in resistance training.
Based upon the results of the studies reviewed in this article, the time-of-day effect on physical performances is a factor that must be taken into account by coaches, athletes, and sports scientists. In fact, although results regarding the effect of time of day on aerobic performances are conflicting, the diurnal rhythm of anaerobic performances has been shown in several studies with morning nadirs and afternoon-evening peak values. In the main, circadian rhythms of sports performances are linked closely in phase with the rhythm of core body temperature suggesting a causal link between them. These diurnal variations are influenced by the regular training at a particular time of day. Indeed, based on previous published studies, training in the morning can (a) improve typically poor morning performances to the same or even higher level as their normal daily peak typically observed in the late afternoon and (b) decrease the amplitude of the diurnal rhythm. However, training in the evening hours can increase the amplitude of the daily variations of neuromuscular performances. However, although Sedliak et al. (102–104) examined the effect of resistance training at a specific time of day on hormonal concentrations of testosterone and cortisol, neuromuscular and muscle CSA adaptations, the exact underlying mechanisms are yet unknown. Current data from the studies of Sedliak and coworkers do not enable changes to be attributed to modifications in the neuroendocrine system. However, because adaptations to resistance training are dependent on the duration and several factors of the acute variables of the training program (e.g., training frequencies, number of sets, rest period between sets, type of exercise), further research is needed to clarify the issue of the temporal specificity of training.
For practical considerations, athletes required to compete at a certain time of day (i.e., when the time of competition is known) may be advised to coincide training hours with the time of day at which one's critical performance is planned. If this is impractical (i.e., the time of competition in not known) resistance training program should be scheduled in the morning to minimize the time-of-day effect on physical performances.
1. Ahtiainen JP, Pakarinen A, Alen M, Kraemer WJ, Häkkinen K. Muscle hypertrophy, hormonal adaptations and strength development during strength training in strength-trained and untrained men. Eur J Appl Physiol 89: 555–563, 2003.
2. Atkinson G, Coldwells A, Reilly T, Waterhouse JM. A comparison of circadian rhythms in work performance between physically active and inactive subjects. Ergonomics 36: 273–281, 1993.
3. Atkinson G, Holder A, Robertson C, Gant N, Drust B, Reilly T, Waterhouse J. Effects of melatonin on the thermoregulatory responses to intermittent exercise. J Pineal Res 39: 353–359, 2005.
4. Atkinson G, Reilly T. Circadian variation in sports performance. Sports Med 21: 292–312, 1996.
5. Bergh U, Ekblom B. Influence of muscle temperature on maximal muscle strength and power output in human skeletal muscles. Acta Physiol Scand 107: 33–37, 1979.
6. Bernard T, Giacomoni M, Gavarry O, Seymat M, Falgairette G. Time-of-day effects in maximal anaerobic leg exercise. Eur J Appl Physiol Occup Physiol 77: 133–138, 1998.
7. Bessot N, Moussay S, Clarys JP, Gauthier A, Sesboue B, Davenne D. The influence of circadian rhythm on muscle activity and efficient force production during cycling at different pedal rates. J Electromyogr Kinesiol 17: 176–183, 2007.
8. Bessot N, Moussay S, Gauthier A, Larue J, Sesboüé B, Davenne D. Effects of pedal rate on diurnal variations in cardiorespiratory variables. Chronobiol Int 23: 877–887, 2006.
9. Bird SP, Tarpenning KM. Influence of circadian time structure on acute hormonal responses to a single bout of heavy-resistance exercise in weight-trained men. Chronobiol Int 21: 131–146, 2004.
10. Blonc S, Perrot S, Racinais S, Aussepe S, Hue O. Effects of 5 weeks of training at the same time of day on the diurnal variations of maximal muscle power performance. J Strength Cond Res 24: 23–29, 2010.
11. Brisswalter J, Bieuzen F, Giacomoni M, Tricot V, Falgairette G. Morning-to-evening differences in oxygen uptake kinetics in short-duration cycling exercise. Chronobiol Int 24: 495–506, 2007.
12. Callard D, Davenne D, Gauthier A, Lagarde D, Van Hoecke J. Circadian rhythms in human muscular efficiency: Continuous physical exercise versus continuous rest. A crossover study. Chronobiol Int 17: 693–704, 2000.
13. Cappaert T. Time of day effect on athletic performance: An update. J Strength Cond Res 13: 412–421, 1999.
14. Carter H, Jones AM, Maxwell NS, Doust JH. The effect of interdian and diurnal variation on oxygen uptake kinetics during treadmill running. J Sports Sci 20: 901–909, 2002.
15. Castaingts V, Martin A, Van Hoecke J, Perot C. Neuromuscular efficiency of the triceps surae in induced and voluntary contractions: Morning and evening evaluations. Chronobiol Int 21: 631–643, 2004.
16. Chtourou H, Chaouachi A, Driss T, Dogui M, Behm DG, Chamari K, Souissi N. The effect of training at the same time of day and tapering period on the diurnal variation of short exercise performances. J Strength Cond Res 26: 697–708, 2012.
17. Chtourou H, Chaouachi A, Hammouda O, Chamari K, Souissi N. Listening to music affects diurnal variation in muscle power output. Int J Sports Med 33: 43–47, 2012.
18. Chtourou H, Driss T, Souissi S, Gam A, Chaouachi A, Souissi N. The effect of strength training at the same time of the day on the diurnal ﬂuctuations of muscular anaerobic performances. J Strength Cond Res 26: 217–225, 2012.
19. Chtourou H, Hammouda O, Chaouachi A, Chamari K, Souissi N. The effect of time-of-day and Ramadan fasting on anaerobic performances. Int J Sports Med 33: 142–147, 2012.
20. Chtourou H, Hammouda O, Souissi H, Chamari K, Chaouachi A, Souissi N. The effect of Ramadan fasting on physical performances, mood state and perceived exertion in young footballers. Asian J Sports Med 2: 177–185, 2011.
21. Chtourou H, Zarrouk N, Chaouachi A, Dogui M, Behm DG, Chamari K, Hug F, Souissi N. Diurnal variation in Wingate-test performance and associated electromyographic parameters. Chronobiol Int 28: 706–713, 2011.
22. Coldwells A, Atkinson G, Reilly T. Sources of variation in back and leg dynamometry. Ergonomics 37: 79–86, 1994.
23. Conroy RT, O'Brien M. Diurnal variation in athletic performance. J Physiol 236: 51, 1974.
24. Dalton B, McNaughton L, Davoren B. Circadian rhythms have no effect on cycling performance. Int J Sports Med 18: 538–542, 1997.
25. Deschenes MR, Sharma JV, Brittingham KT, Casa DJ, Armstrong LE, Maresh CM. Chronobiological effects on exercise performance and selected physiological responses. Eur J Appl Physiol 77: 249–256, 1998.
26. Dinneen S, Alzaid A, Miles J, Rizza R. Metabolic effects of the nocturnal rise in cortisol on carbohydrate-metabolism in normal humans. J Clin Invest 92: 2283–2290, 1993.
27. Drust B, Waterhouse J, Atkinson G, Edwards B, Reilly T. Circadian rhythms in sports performance-an update. Chronobiol Int 22: 21–44, 2005.
28. Edwards BJ, Edwards W, Waterhouse J, Atkinson G, Reilly T. Can cycling performance in an early morning, laboratory-based cycle time-trial be improved by morning exercise the day before. Int J Sports Med 26: 651–656, 2005.
29. Faria JE, Drummond BJ. Circadian changes in resting heart rate and body temperature, maximal oxygen consumption and perceived exertion. Ergonomics 25: 381–386, 1982.
30. Ferrando AA, Tipton KD, Doyle D, Phillips SM, Cortiella J, Wolfe RR. Testosterone injection stimulates net protein synthesis but not tissue amino acid transport. Am J Physiol 275: 864–871, 1998.
31. Florini JR. Hormonal-control of muscle growth. Muscle Nerve 10: 577–598, 1987.
32. Forsyth JJ, Reilly T. Circadian rhythms in blood lactate concentration during incremental ergometer rowing. Eur J Appl Physiol 92: 69–74, 2004.
33. Fryburg DA, Barrett EJ. Growth hormone acutely stimulates skeletal muscle but not whole-body protein synthesis in humans. Metabolism 42: 1223–1227, 1993.
34. Gauthier A, Davenne D, Gentil C, Van Hoecke J. Circadian rhythm in the torque developed by elbow flexors during isometric contraction. Effect of sampling schedules. Chronobiol Int 14: 287–294, 1997.
35. Gauthier A, Davenne D, Martin A, Cometti G, Van Hoecke J. Diurnal rhythm of the muscular performance of elbow flexors during isometric contractions. Chronobiol Int 13: 135–146, 1996.
36. Gauthier A, Davenne D, Martin A, Van Hoecke J. Time-of-day effects on isometric and isokinetic torque developed during elbow flexion in humans. Eur J Appl Physiol 84: 249–252, 2001.
37. Giacomoni G, Edwards B, Bambaeichi E. Gender differences in the circadian variations in muscle strength assessed with and without superimposed electrical twitches. Ergonomics 48: 147–148, 2005.
38. Giacomoni M, Benard T, Gavarry O, Altare S, Fulgairette G. Dirunal variations in ventilatory and cardiorespiratory responses to submaximal treadmill exercise in females. Eur J Appl Physiol 80: 591–597, 1999.
39. Guette M, Gondin J, Martin A. Time-of-day effect on the torque and neuromuscular properties of dominant and non-dominant quadriceps femoris. Chronobiol Int 22: 541–558, 2005.
40. Häkkinen K, Komi PV, Alen M. Effect of explosive type strength training on isometric force- and relaxation-time, electro-myographic and muscle ﬁbre characteristics of leg extensor muscles. Acta Physiol Scand 125: 587–600, 1985.
41. Häkkinen K, Pakarinen A, Alen M, Kauhanen H, Komi PV. Daily hormonal and neuromuscular responses to intensive strength training in 1 week. Int J Sports Med 9: 422–428, 1988.
42. Hakkinen K, Pakarinen A, Kraemer WJ, Hakkinen A, Valkeinen H, Alen M. Selective muscle hypertrophy, changes in EMG and force, and serum hormones during strength training in older women. J Appl Physiol 91: 569–580, 1988.
43. Häkkinen K, Pakarinen A, Newton RU, Kraemer WJ. Acute hormonal responses to heavy resistance lower and upper extremity exercise in young versus old men. Eur J Appl Physiol 77: 312–319, 1998.
44. Hammouda O, Chtourou H, Chahed H, Ferchichi S, Chaouachi A, Kallel C, Miled A, Chamari K, Souissi N. High intensity exercise affects diurnal variation of some biological markers in trained subjects. Int J Sports Med. 33: 1–6, 2012.
45. Hammouda O, Chtourou H, Chahed H, Ferchichi S, Kallel C, Miled A, Chamari K, Souissi N. Diurnal variations of plasma homocysteine, total antioxidant status and biological markers of muscle injury during repeated sprint: Effect on performance and muscle fatigue: A pilot study. Chronobiol Int 28: 958–967, 2011.
46. Hamouda O, Chtourou H, Farjallah MA, Davenne D, Souissi N. The effect of Ramadan fasting on the diurnal variations in aerobic and anaerobic performances in Tunisian youth soccer players. Biol Rhythms Res 43: 177–190, 2012.
47. Hayes LD, Bickerstaff GF, Baker JS. Interactions of cortisol, testosterone, and resistance training: Influence of circadian rhythms. Chronobiol Int 27: 675–705, 2010.
48. Hill DW, Cuerton KJ, Collins MA. Circadian specificity in exercise training. Ergonomics 32: 79–92, 1989.
49. Hill DW, Cureton KJ, Collins MA, Grisham SC. Diurnal variations in responses to exercise of “morning types” and “evening types.” J Sports Med 28: 213–219, 1988.
50. Hill DW, Leiferman JA, Lynch NA, Dangelmaier BS, Brut SE. Temporal specificity in adaptations to high intensity exercise training. Med Sci Sports Exerc 30: 450–455, 1998.
51. Ilmarinen J, Ilmarinen R, Korhonen O, Nurminen M. Circadian variation of physiological functions related to physical work capacity. Scand J Work Environ Health 6: 112–122, 1980.
52. Jasper I, Haussler A, Baur B, Marquardt C, Hermsdorfer J. Circadian variations in the kinematics of handwriting and grip strength. Chronobiol Int 26: 576–594, 2009.
53. Jones H, George K, Edwards B, Atkinson G. Effects of time of day on post-exercise blood pressure: Circadian or sleep-related influences? Chronobiol Int 25: 987–998, 2008.
54. Jones H, Pritchard C, George K, Edwards B, Atkinson G. The acute post-exercise response of blood pressure varies with time of day. Eur J Appl Physiol 104: 481–489, 2008.
55. Jones H, Taylor CE, Lewis NC, George K, Atkinson G. Post-exercise blood pressure reduction is greater following intermittent than continuous exercise and is influenced less by diurnal variation. Chronobiol Int 26: 293–306, 2009.
56. Jones HA, Hill DW. Responses to incremental and constant power exercise tests at different times of day. Med Sci Sports Exerc 27: 109, 1995.
57. Kadi F. Adaptation of human skeletal muscle to training and anabolic steroids. Acta Physiol Scand Suppl 646: 1–52, 2000.
58. Kario K, Pickering TG, Umeda Y, Hoshide S, Hoshide Y, Morinari M, Murata M, Kuroda T, Schwartz JE, Shimada K. Morning surge in BP as a predictor of silent and clinical cerebrovascular disease in elderly hypertensives: A prospective study. Circulation 107: 1401–1406, 2004.
59. Kin-Isler A. Time-of-day effects in maximal anaerobic performance and blood lactate concentration during and after a supramaximal exercise. Iso Exerc Sci 14: 335–340, 2006.
60. Kraemer WJ. Endocrine responses to resistance exercise. Med Sci Sport Exerc 20: 152–157, 1988.
61. Kraemer WJ, Gordon SE, Fleck SJ, Marchitelli LJ, Mello R, Dziados JE, Friedl K, Harman E, Maresh C, Fry AC. Endogenous anabolic hormonal and growth factor responses to heavy resistance exercise in males and females. Int J Sports Med 12: 228–235, 1991.
62. Kraemer WJ, Loebel CC, Volek JS, Ratamess NA, Newton RU, Wickham RB, Gotshalk LA, Duncan ND, Mazzetti SA, Gomez AL, Rubin MR, Nindl BC, Häkkinen K. The effect of heavy resistance exercise on the circadian rhythm of salivary testosterone in men. Eur J Appl Physiol 84: 13–18, 2001.
63. Kraemer WJ, Marchitelli L, Gordon SE, Harman E, Dziados JE, Mello R, Frykman P, McCurry D, Fleck SJ. Hormonal and growth factor responses to heavy resistance exercise protocols. J Appl Physiol 69: 1442–1450, 1990.
64. Kraemer WJ, Ratamess NA. Hormonal responses and adaptations to resistance exercise and training. Sports Med 35: 339–361, 2005.
65. Kvorning T, Andersen M, Brixen K, Schjerling P, Suetta C, Madsen K. Suppression of testosterone does not blunt mRNA expression of myoD, myogenin, IGF, myostatin or androgen receptor post strength training in humans. J Physiol 15: 579–593, 2007.
66. Lappalainen Z, Kilinc F, Lappalainen J, Atalay M. Time-of-day effects during acute isokinetic exhaustive eccentric exercise: Serum leptin response. Isokinet Exerc Sci 17: 19–25, 2009.
67. Lericollais R, Gauthier A, Bessot N, Davenne D. Diurnal evolution of cycling biomechanical parameters during a 60-s Wingate test. Scand J Med Sci Sports 10: 1–10, 2010.
68. Lericollais R, Gauthier A, Bessot N, Sesboüé B, Davenne D. Time-of-day effects on fatigue during a standard anaerobic test in well-trained cyclists. Chronobiol Int 26: 1622–1635, 2009.
69. Linnamo V, Pakarinen A, Komi PV, Kraemer WJ, Häkkinen K. Acute hormonal responses to submaximal and maximal high intensity resistance and explosive exercise in men and women. J Strength Cond Res 19: 566–571, 2005.
70. Martin A, Carpentier A, Guissard N, Van Hoecke J, Duchateau J. Effect of time of day on force variation in a human muscle. Muscle Nerve 22: 1380–1387, 1999.
71. McCall GE, Byrnes WC, Fleck SJ, Dickinson A, Kraemer WJ. Acute and chronic hormonal responses to resistance training designed to promote muscle hypertrophy. Can J Appl Physiol 24: 96–107, 1999.
72. Melhim AF. Investigation of circadian rhythms in peak power and mean power of female physical education students. Int J Sports Med 14: 303–306, 1993.
73. Morris C, Atkinson G, Drust B, Marrin K, Gregson W. Human core temperature responses during exercise and subsequent recovery: An important interaction between diurnal variation and measurement site. Chronobiol Int 26: 560–575, 2009.
74. Mujika I, Padilla S, Pyne D, Busso T. Physiological changes associated with the pre-event taper in athletes. Sports Med 34: 891–927, 2004.
75. Muller JE, Toﬂer GH, Stone PH. Circadian variation and triggers of onset of acute cardiovascular disease. Circulation 79: 733–743, 1989.
76. Nicolas A, Gauthier A, Bessot N, Moussay S, Davenne D. Time-of-day effects on myoelectric and mechanical properties of muscle during maximal and prolonged isokinetic exercise. Chronobiol Int 22: 997–1011, 2005.
77. Nicolas A, Gauthier A, Bessot N, Moussay S, Thibault G, Sesboüé B, Davenne D. Effect of time-of-day on neuromuscular properties of knee extensors after a short exhaustive cycling exercise. Iso Exerc Sci 16: 33–40, 2008.
78. Nicolas A, Gauthier A, Michaut A, Davenne D. Effect of circadian rhythm of neuromuscular properties on muscle fatigue during concentric and eccentric isokinetic actions. Isokinetics Exerc Sci 15: 117–129, 2007.
79. Onambele-Pearson G, Pearson S. Time-of-day effect on patella tendon stiffness alters vastus lateralis fascicle length but not the quadriceps force–angle relationship. J Biomech 40: 1031–1037, 2007.
80. Pearson S, Bruce S, Graham-Smith P, Onambele G. Human knee-extensors architecture: Diurnal rhythmicity and torque characteristics. J Physiol 565: 113, 2004.
81. Pearson SJ, Onambele GN. Acute changes in knee-extensors torque, fiber pennation, and tendon characteristics. Chronobiol Int 22: 1013–1027, 2005.
82. Peeters GM, Van Schoor NM, Van Rossum EF, Visser M, Lips P. The relationship between cortisol, muscle mass and muscle strength in older persons and the role of genetic variations in the glucocorticoid receptor. Clin Endocrinol 69: 673–682, 2008.
83. Racinais S. Different effects of heat exposure upon exercise performance in the morning and afternoon. Scand J Med Sci Sports 20: 80–89, 2010.
84. Racinais S, Blonc S, Hue O. Effects of active warm-up and diurnal increase in temperature on muscular power. Med Sci Sports Exerc 37: 2134–2139, 2005.
85. Racinais S, Blonc S, Jonville S, Hue O. Time-of-day influences the environmental effects on muscle force and contractility. Med Sci Sports Exerc 37: 256–261, 2005.
86. Racinais S, Chamari K, Hachana Y, Bartagi Z, Blonc S, Hue O. Effect of an acute hot and dry exposure in moderately warm and humid environment on muscle performance at different time-of-day. Int J Sports Med 27: 49–54, 2006.
87. Racinais S, Connes P, Bishop D, Blonc S, Hue O. Morning versus evening power output and repeated-sprint ability. Chronobiol Int 22: 1029–1039, 2005.
88. Racinais S, Hue O, Blonc S. Time-of-day effects on anaerobic muscular power in a moderately warm environment. Chronobiol Int 21: 485–495, 2004.
89. Racinais S, Hue O, Hertogh C, Damiani M, Blonc S. Time-of-day effects in maximal anaerobic leg exercise in tropical environment: A first approach. Int J Sports Med 25: 186–190, 2004.
90. Racinais S, Perrey S, Denis R, Bishop D. Maximal power, but not fatigability, is greater during repeated sprints performed in the afternoon. Chronobiol Int 27: 855–864, 2010.
91. Reilly T. Circadian variation in ventilatory and metabolic adaptations to submaximal exercise. Br J Sports Med 16: 115–116, 1982.
92. Reilly T. Human circadian rhythms and exercise. Crit Rev Biomed Eng 18: 165–180, 1990.
93. Reilly T, Atkinson G, Edwards B, Waterhouse J, Farrelly K, Fairhurst E. Diurnal variation in temperature, mental and physical performance, and tasks specifically related to football (soccer). Chronobiol Int 24: 507–519, 2007.
94. Reilly T, Bambaeichi E. Methodological issues in studies of rhythms in human performance. Biol Rhythm Res 34: 321–336, 2003.
95. Reilly T, Baxter C. Influence of time of day on reactions to cycling at a fixed high intensity. Br J Sports Med. 17: 128–130, 1983.
96. Reilly T, Brooks GA. Investigation of circadian rhythms in metabolic responses to exercise. Ergonomics 25: 1093–1097, 1982.
97. Reilly T, Brooks GA. Selective persistence of circadian rhythms in physiological responses to exercise. Chronobiol Int 7: 59–67, 1990.
98. Reilly T, Garrett R. Effects of time of day on self-paced performances of prolonged exercise. J Sports Med Phys Fitness 35: 99–102, 1995.
99. Reilly T, Garrett R. Investigation of diurnal variation in sustained exercise performance. Ergonomics 41: 1085–1094, 1998.
100. Reilly T, Waterhouse J. Chronobiology and exercise. Med Sport 13: 54–60, 2009.
101. Sedliak M, Finni T, Cheng S, Haikarainen T, Hakkinen K. Diurnal variation in maximal and submaximal strength, power and neural activation of leg extensors in men: Multiple sampling across two consecutive days. Int J Sports Med 29: 217–224, 2008.
102. Sedliak M, Finni T, Cheng S, Lind M, Häkkinen K. Effect of time-of-day-specific strength training on muscular hypertrophy in men. J Strength Cond Res 23: 2451–2457, 2009.
103. Sedliak M, Finni T, Peltonen J, Häkkinen K. Effect of time-of-day-specific strength training on maximum strength and EMG activity of the leg extensors in men. J Sports Sci 26: 1005–1014, 2008.
104. Sedliak M, Finni T, Sulin C, Kraemer WJ, Häkkinen K. Effect of time-of-day-specific strength training on serum hormone concentrations and isometric strength in men. Chronobiol Int 24: 1159–1177, 2007.
105. Souissi H, Chaouachi A, Chamari K, Dogui M, Amri M, Souissi N. Time-of-day effects on short-term exercise performances in 10-11-years-old boys. Pediatr Exerc Sci 22: 613–623, 2010.
106. Souissi H, Chtourou H, Chaouachi A, Dogui M, Chamari K, Amri M, Souissi N. The effect of training at a specific time-of-day on the diurnal variations of short-term exercise performances in 10-11-years-old Boys. Pediatr Exerc Sci 24: 84–99, 2012.
107. Souissi M, Chtourou H, Zrane A, Ben Cheikh R, Dogui M, Tabka Z, Souissi N. Effect of time-of-day of aerobic maximal exercise on the sleep quality of trained subjects. Biol Rhythm Res 2012, DOI:10.1080/09291016.2011.589159.
108. Souissi N, Bessot N, Chamari K, Gauthier A, Sesboüé B, Davenne D. Effect of time of day on aerobic contribution to the 30-s Wingate test performance. Chronobiol Int 24: 739–748, 2007.
109. Souissi N, Driss T, Chamari K, Vandewalle H, Davenne D, Gam A, Fillard JR, Jousselin E. Diurnal variation in Wingate test performances: Influence of active warm-up. Chronobiol Int 27: 640–652, 2010.
110. Souissi N, Gauthier A, Sesboüé B, Larue J, Davenne D. Circadian rhythms in two types of anaerobic cycle leg exercise: Force-velocity and 30-s Wingate tests. Int J Sports Med 25: 14–19, 2004.
111. Souissi N, Gauthier A, Sesboüé B, Larue J, Davenne D. Effects of regular training at the same time of day on diurnal fluctuations in muscular performance. J Sports Sci 20: 929–937, 2002.
112. Souissi N, Sesboue B, Gauthier A, Larue J, Davenne D. Effects of one night's sleep deprivation on anaerobic performance the following day. Eur J Appl Physiol 89: 359–366, 2003.
113. Souissi N, Souissi M, Souissi H, Chamari K, Tabka Z, Dogui M, Davenne D. Effect of time of day and partial sleep deprivation on short-term, high-power output. Chronobiol Int 25: 1062–1076, 2008.
114. Taylor K, Cronin JB, Gill N, Chapman DW, Sheppard JM. Warm-up affects diurnal variation in power output. Int J Sports Med 32: 185–189, 2011.
115. Torii J, Shinkai S, Hino S. Effect of time of day on adaptive response to a 4-week aerobic exercise program. J Sports Med Phys Fitness 32: 348–352, 1992.
116. Touitou Y, Haus E. Alterations with aging of the endocrine and neuroendocrine circadian system in humans. Chronobiol Int 17: 369–390, 2000.
117. Vingren JL, Kraemer WJ, Hatfield DL, Volek JS, Ratamess NA, Anderson JM, Häkkinen K, Ahtiainen J, Fragala MS, Thomas GA, Ho JY, Maresh CM. Effect of resistance exercise on muscle steroid receptor protein content in strength-trained men and women. Steroids 74: 1033–1039, 2009.
118. Waterhouse J, Reilly T, Atkinson G, Edwards B. Jet lag: Trends and coping strategies. Lancet 369: 1117–1129, 2007.
119. Williams CS, Hill DW. The effect of time of day on VO2 kinetics and ·VO2 max during high-intensity exercise. Med Sci Sports Exerc 27: 109, 1995.
120. Wyse JP, Mercer TH, Gleeson NP. Time-of-day dependence of isokinetic leg strength and associated interday variability. Br J Sports Med 28: 167–170, 1994.