Introduction
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
Short-Duration Exercises
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).
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Table 1-a: The effect of time of day on short-term maximal performances.*
Table 1-b: The effect of time of day on short-term maximal performances.*
Table 1-c: The effect of time of day on short-term maximal performances.*
Table 1-d: The effect of time of day on short-term maximal performances.*
Table 1-e: The effect of time of day on short-term maximal performances.*
Table 1: f The effect of time of day on short-term maximal performances.*
Table 1: g The effect of time of day on short-term maximal performances.*
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).
Figure 1: Circadian rhythm in core temperature, peak (
P peak) and mean (
P mean) powers during the Wingate test, and maximal power (
P max) during the force-velocity test. Drawn from data presented by Souissi et al. (
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.
Table 2: The effect of time of day on long-duration exercises performances.*
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 [100]).
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.
Table 3: Circadian rhythm in resting physiological variables (
91,98).
*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).
Table 4-a: The effect of time of day on the physiological responses to long-duration exercises.*
Table 4-b: The effect of time of day on the physiological responses to long-duration exercises.*
Table 4-c: The effect of time of day on the physiological responses to long-duration exercises.*
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 [100]). 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
Aerobic Training
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).
Resistance 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.
Table 5: Studies on the effect of time-of-day specific training on short-term maximal performances.*
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.
Neuromuscular Adaptations
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.
Conclusions
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.
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