One of the basic fields in chronobiology science is to determine the circadian rhythm of physiological variables which refer to the variations that recur every 24 hours. In this context, many controlled biological functions have shown to be time-of-day (TOD) dependents, and their physiological circadian rhythms are well documented in healthy populations at resting state. Indeed, it is well established in healthy active adults that markers of oxidative stress and antioxidant status are TOD-dependent (32,37,38) with more efficient antioxidant system in the early morning and greater rate of lipid peroxidation in the early evening (35,37). In this way, Borisenkov et al. (12), Hardeland et al. (32), Kanabrocki (38), and Singh et al. (59) showed a higher oxidative stress in the evening with peak concentrations of malondialdehyde (MDA) occurred early in the evening at 18:00 hours (i.e., lower values at 06:00 hours) and higher values of antioxidant status parameters (i.e., total antioxidant status [TAS], total bilirubin [TBIL], uric acid [UA], catalase [CAT], and glutathione peroxidase [GPX] activities) in the early morning with peak concentrations at 06:00 hours (i.e., which decrease gradually to a minimum at 18:00 hours and at 00:00 hours, respectively, for CAT and GPX). Similarly, in healthy sedentary subjects, resting levels of biochemical markers of muscle damage and fatigue (e.g., creatine kinase CK), lactate dehydrogenase (LDH), alanine aminotransferase (ALAT), aspartate aminotransferase (ASAT), gamma-glutamyl transpeptidase (γ-GT), phosphatase alkaline (PAL), and blood lactate (Lac) are TOD-dependent (27,38,39,56) with significantly higher values in the early evening compared with those in the morning (i.e., peak values observed at 17:00 hours).
Resting values of white blood cell (WBC) and their subpopulations (monocytes (MO), neutrophils (NE), and lymphocytes (LY) were significantly higher in the early evening (i.e., ∼18:00 hours, close to the acrophase of oral temperature (56)) than in the morning (26,27,38) in healthy sedentary subjects. In addition, resting plasma levels of urea (URE), creatinine (CR), glucose (GLC), total cholesterol (TC), triglycerides (TG), and high-density lipoprotein (HDL) were higher in the afternoon than in the morning (38,39).
The circadian rhythms of the abovementioned physiological variables are well established in healthy adults at resting state. However, given the interaction between both TOD and physical exercise effects on the responses of these biochemical parameters, the circadian rhythm of theses parameters are still not well established in trained subjects during physical activity (3,4,28–30). In this context, after physical exercise, previous reports showed that whatever the TOD, the majority of the physiological functions increase from pre to posttraining session (3,4,28–30) with different rates of increases from the morning to the evening. These differences in the increasing rates of the biochemicals during physical tasks may alter, in postexercise, their diurnal variations observed in the resting state (3,4). Otherwise, it was suggested that the diurnal variation of these biochemical parameters could, in part, explain the typical diurnal fluctuation of physical performance. Indeed, previous reports showed a similar diurnal variation of short-term maximal performance recorded during the repeated sprint exercise (RSA) and the 30-s Wingate tests (17,18). For example, during the 30-s Wingate test, Chtourou et al. (17) reported that peak and mean power were significantly higher at 17:00 hours than at 07:00 hours. Moreover, the diurnal fluctuations of sport performances and the abovementioned biochemical parameters were observed to be close to that of core temperature (4).
Consequently, given that the effect of the physical exercise on the biochemical parameters' diurnal variation still not well understood and given that the relationship between the circadian rhythms of the biological functions and the physical performance still not well established, the aims of this study were: (a) to review the effect of TOD on the biochemical responses during physical exercise, (b) to discuss the possible interaction between these variables, and (c) to establish the possible link between the diurnal variation of sport performance, biochemical parameters, and core temperature.
To that end, we searched—up to January 2016—the electronic databases PubMed, ISI Web of Knowledge, SPORTDiscus, and Google Scholar. We included and combined “sport performance,” “biochemical response,” “diurnal variation,” “Time-of-day,” “physical exercise” as key terms. Only scientific research using accepted methods that provided relevant information about diurnal variation of physiological variables at rest and in response to physical exercise were included in the current review.
Effect of Time of Day on Biochemical Parameters at Rest and During Physical Exercise
The results of previous studies concerning the effect of TOD on hematological, oxidative stress, and muscle damage parameters are presented in Table 1.
In healthy sedentary subjects (33,38), resting values of WBC and their subpopulations (MO, NE, and LY) were significantly higher in the afternoon (i.e., ∼18:00 hours) than in the morning. This resting diurnal variation has also been reported in trained subjects. In fact, Hammouda et al. (28–30) found similar results in male young soccer players and showed that, at resting state, the values of the hematological parameters increase significantly (i.e., p ≤ 0.05 for WBC, p < 0.01 for LY and MO and p < 0.001 for NE) from the morning (07:00 hours) to the early evening (17:00 hours). Similarly, in elite weightlifters, Ammar et al. (4) have investigated the diurnal variation (i.e., 08:00 hours, 14:00 hours, and 18:00 hours) of the same parameters and reported higher resting values of WBC (p < 0.01) and NE (p < 0.001) in the early evening (18:00 hours) than in the morning. These higher values are mainly due to the significant increase between the afternoon and the evening. In fact, significant differences (p < 0.01) were shown between 14:00 hours and 18:00 hours for these parameters.
Immediately after physical exercise, a transient increase of the hematological parameters (e.g., WBC and red blood cell (RBC)) was reported (4,28–30) from pre to post exercise. Similarly, Yalcin et al. (70) indicated that RBC, hematocrit (HCT), and WBC (as well as their subpopulations) increased in the morning and in the afternoon after a physical exercise. Despite these increases, the diurnal variations persisted for the majority of these parameters at postexercise. Indeed, previous studies reported a significant TOD effect on WBC, NE, LYM, and MON with higher values in the early evening after an RSA exercise (29), a 30-s Wingate test (28), and a weightlifting training session (4). In addition, a significant interaction TOD × physical exercise was shown for the majority of these parameters (p ≤ 0.05 for WBC, NE, LY, and MO), which (a) reflects the tendencies for higher values of these parameters after exercise at 17:00 hours than at 07:00 hours (29) and (b) explain the fact that the resting fluctuation morning-evening persisted and even increased after physical exercise. In this context, Hammouda et al. (28–30) concluded that exercise-induced leukocytosis was higher in the afternoon and speculated that the higher hematological levels could be due to the higher catecholamine concentrations at this TOD (38). This exercise-induced leukocytosis could possibly affect the diurnal variation in muscle fatigue and performance during short-term maximal exercise. Indeed, it has been reported that a 10 × 6-s RSA exercise with 30 seconds of recovery in between induced 18-fold increase in plasma adrenaline (14).
Concerning RBC and HCT, in trained subjects, the resting values of these parameters were higher in the morning than in the evening (28–30). However, these diurnal fluctuations were blunted for RBC after both short- and long-duration exercises (28–30) or weightlifting training session (4) and persist for HCT after a 30-s Wingate test (28).
Selected Biochemical Parameters
In both sedentary and trained subjects, data of the literature (Table 1) showed that resting plasma levels of URE, CR, GLC, TC, TG, and HDL are TOD-dependent with higher levels in the evening than in the morning (28–30,38,39). All these biochemical variables were raised after the exercise with respect to pre-exercise at the different TOD, with higher postexercise values observed in the afternoon (i.e., URE, CR, GLC after the RSA and the Wingate exercise (28–30) and GLC, TC, TG, and HDL after the Yo-Yo test (30)). This higher increase during the afternoon physical exercise may indicate an enhanced lipid and glucose mobilization at this TOD that could be explained by their diurnal variation observed in the resting state. Indeed, previous researchers reported that free and esterified cholesterol of low-density lipoprotein and HDL showed a significant diurnal variation with the lowest values observed in the morning followed by an increase before breakfast and the highest levels during the afternoon (15,46). However, the greater postexercise GLC levels were suggested to be a result of an increased hepatic glucose production, which exceeds the concomitant rise in peripheral glucose uptake, as shown during high-intensity exercise (42). The greater postexercise CR levels indicated its strong mobilization (recycled or converted to CR), higher buffering capacity, and phosphocreatine replenishment, which are the most important determinants of the ability to repeat sprints (7).
Concerning the effect of these parameters on performance during a long-duration exercise (i.e., Yo-Yo test), previous studies suggest a possible link between the diurnal fluctuation of metabolic responses and the related pattern of specific-endurance performances in soccer players (28,29). Furthermore, the significant interaction between TOD × before/after RSA or Wingate test observed in the studies of Hammouda et al. (28,29) for some biochemical parameters suggests that the evening session is associated with greater levels of glycolysis (i.e., with higher levels of Lac and GLC). Therefore, the higher biochemical responses observed in the evening could explain, partially, the greater performance and metabolic solicitation at 17:00 hours test session (e.g., for RSA, Wingate or Yo-Yo test) (28–30).
In trained subjects, plasma levels of homocysteine (Hcy) was observed to be higher in the early evening than in the morning (11,29). The high level of Hcy observed in the early evening was maintained after physical exercise (i.e., RSA testing) (29), which may indicate that Hcy values tended to be slightly higher when exercise was performed at 17:00 hours. This greater Hcy mobilization after the early evening exercise (Table 1) could be due to the similar increase in CR levels. Indeed, Hcy metabolism is, also, muscularly related to CR levels (34). In this context, Di Santolo et al. (22) have found a positive correlation between Hcy and CR levels. Indeed, CR synthesis in the liver accounts for approximately 75% of daily Hcy formation (58) where S-adenosylmethionine donates its methyl group to guanidinoacetate for CR and S-adenosylhomocysteine creation.
Muscle Damage and Lactate
Previous reports (Table 1) have shown that, in healthy sedentary subjects, resting levels of biochemical markers of muscle damage and fatigue (e.g., CK, LDH, ALAT, ASAT, γ-GT, PAL) as well as Lac are TOD-dependent (27,38,39,56) with acrophases (i.e., peak values) observed at 17:00 hours. In this context, Racinais et al. (52) and Hammouda et al. (28) have shown a similar diurnal variation in soccer players. In a resting state, Lac levels and markers of muscle injury were greater in the afternoon (e.g., 148.7 ± 67.05 IU·L−1 vs. 195 ± 74.6 IU·L−1 for CK). Similarly, Ammar et al. (4) reported a diurnal variation of the majority of muscle damage' markers in elite weightlifters and showed that, in resting state, the diurnal increase morning to evening of these parameters (i.e., p < 0.01 for CK, p ≤ 0.05 for ASAT and p < 0.001 for LDH, GT, and PAL) is generally observed between 08:00 hours and 14:00 hours; however, no significant difference was observed between 14:00 hours and 17:00 hours. In sedentary subjects, the diurnal variations of resting values have been reported to be linked to the circadian rhythm of core temperature (33,56). Indeed, the afternoon acrophases of Lac and studied enzymes coincide with the time peak of oral temperature (56). Dalton et al. (20) and Hammouda et al. (28) confirmed this relation temperature-biochemical levels in trained subjects. In this context, Dalton et al. (20) suggested that circadian changes in core temperature would increase enzymatic activities such as phosphofructokinase and LDH, which could in turn increase the GLC and Lac production. However, Hammouda et al. (28) showed that oral temperature and resting plasma levels of enzymes were both higher at 17:00 hours in soccer players.
After physical tasks, the diurnal fluctuations of muscle damage' markers persisted after the RSA (29), the Wingate (28), and the Yo-Yo exercise (30) with higher posttask values observed at 17:00 hours than at 07:00 hours (e.g., 191.6 ± 79.52 vs. 263.6 ± 96.06 IU·L−1; 191.18 ± 21.13 vs. 219.27 ± 27.74 IU·L−1 for CK, after the Wingate and the Yo-Yo test, respectively). However, after weightlifting training session (4) the resting diurnal variations for the majority of muscle damage' markers were altered with higher postexercise values of CK, ALAT, and ASAT at 08:00 hours than at 18:00 hours and with no diurnal variations for LDH, PAL, and GT. The posteffort diurnal alteration in these parameters could be explained by the higher rate of increase pre-post training session registered in the morning (4) compared with that in the evening (e.g., 59.91 vs. 25.30% for CK). Similarly, a diurnal variation of Lac responses has been identified during both aerobic (21,25,30) and anaerobic (28) exercises with higher values in the afternoon session, which would be indicative of raised anaerobic metabolic activity at this TOD (64). In addition, during intermittent exercise (29), a significant interaction TOD × Physical exercise was shown for the majority of these parameters (i.e., CK, GT, LDH, ALAT, ASAT, and Lac), suggesting that the rise (morning to evening) in these parameters already shown at resting state was much more marked after physical exercise. Hammouda et al. (29) indicated that the higher increase of these enzymes during the afternoon physical exercises is influenced by their higher resting values and greater initial power output at this TOD. Concerning the Lac, as suggested by Deschenes et al. (21), the morning-evening difference in Lac response to exercise might be explained by the increase in catecholamine activity. Indeed, catecholamines, particularly epinephrine, follow very similar patterns in response to exercise with a peak in the early afternoon and a trough occurring during the night (1). In this context, Racinais et al. (52) and Hammouda et al. (29) suggested that the higher Lac production during the afternoon RSA exercise might explain the higher muscle fatigue at this TOD. Hammouda et al. (30) showed a significant correlation between the total distance covered during the Yo-Yo test and the postexercise Lac levels (i.e., r = 0.69 and p ≤ 0.05). These results confirmed the previous result of Krustrup et al. (40) who showed that this distance was significantly correlated to the rate of Lac accumulations.
Antioxidant Status and Oxidative Stress
Most of the previous studies (Table 1) which examined the diurnal fluctuation of oxidative stress parameters were conducted in healthy sedentary subjects with an average age generally ≥30 years (6,33,37,38,56). These studies reported that, in a resting state, the total antioxidant capacity (12), the activities of some antioxidant enzymes (35), and the rate of lipid peroxidation (37) were TOD-dependent with a more efficient antioxidant system (i.e., responsible in defending organism against the periodic rise in reactive oxygen species) in the early morning (i.e., peaks of CAT, UA, and TBIL recorded at 06:00 hours) and a greater rate of lipid peroxidation in the early evening (peak of MDA occurred at 18:00 hours) (38,59). Although the origins of these fluctuations are unclear, it has been identified that the physiological levels of melatonin contribute to the total antioxidant capacity of human serum (6), as this hormone displays a powerful antioxidant role (63). In addition, these diurnal variations can be explained by the greater increase of Hcy and biomarkers of muscle damage (i.e., CK, LDH, and ASAT) observed in the afternoon. Indeed, these enzymes have been shown to be indicative of free radical production and to be highly correlated with lipid peroxidation at rest (67) and could reflect a more pronounced oxidative stress (37). In addition, it has been hypothesized that the diurnal variation of total antioxidant capacity (12) and low-molecular weight antioxidants (32) are closely correlated with the circadian system of body, respiratory, and motor activities (12,32). Concerning the resting diurnal variation of oxidant/antioxidant parameters in trained population, only 2 studies were conducted until this date (i.e., investigated soccer players and elite weightlifters) and reported contradictory results. In trained soccer players, Hammouda et al. (28–30) indicated that the diurnal variation of the resting antioxidant activity (i.e., TAS, TBIL, and UA) is similar to the one of sedentary subjects with significantly higher values in the morning compared with that in the early evening (e.g., 1.33 ± 0.19 vs. 1.19 ± 0.14 μmol·L−1 for TAS, respectively, at 07:00 hours and 17:00 hours). However, recently in elite weightlifters, Ammar et al. (3) showed that resting values of UA and the enzymatic antioxidant (i.e., CAT and GPX) were affected by the TOD (p ≤ 0.05) with higher values at 18:00 hours compared with 08:00 hours (i.e., rate of increase: 50.53 ± 19.80%, 50.03 ± 22.55%, and 13.22 ± 5.4%, for the same parameters, respectively). Ammar et al. (3) indicated that the diurnal fluctuations of these parameters are due essentially to the significant increases morning-afternoon (i.e., no significant differences between afternoon and evening values). Indeed, significant increases morning-afternoon were found for these parameters with no changes afternoon-evening (e.g., percentage of increase: 43.83 ± 15.1% vs. 05.90 ± 02.5% for GPX). For the rate of lipid peroxidation (i.e., MDA) and TBIL, no significant TOD effects were observed in this study. This lack of the diurnal variation in MDA (i.e., peak values observed at 18:00 hours in sedentary subjects) was explained (3) by the fact that elite weightlifters have more efficient antioxidant status at this TOD (i.e., higher values of CAT, GPX, and UA in the evening). These alterations in the weightlifters resting oxidant/antioxidant diurnal variation were also explained by the effect of training at a specific TOD (3). Indeed, based on the fact that during the 3 years (i.e., chronic adaptation) which proceed this experimentation, the weightlifting training schedule was based on early evening training (15:30–17:30 hours), the authors in this study suggest that resistance training scheduled at the same TOD (between the afternoon and the evening) could increase antioxidant status and decrease lipid peroxidation at this specific time of training. Furthermore, the absence of lipid peroxidation peak in the evening for elite weightlifters confirm the speculation of Bloomer et al. (9,10) and Subudhi et al. (62) indicating that being experienced resistance trained could result in a lower MDA level.
Otherwise, during physical exercise, the resting diurnal variation has been altered immediately and even 48-h post exercise. Indeed, during the Wingate test, Hammouda et al. (28) reported that TAS, TBIL, and UA increased significantly only in the early evening. This difference is due to their diurnal variations, and no significant difference has been shown between morning and evening postexercise values. Similarly, after intermittent exercise (e.g., RSA and Yo-Yo test), the resting diurnal variations of TAS, TBIL, and UA were suppressed (29,30). In the same way, after weightlifting training session, Ammar et al. (3) showed that 3 min after 3 training sessions performed at different TOD (i.e., 08:00, 14:00, and 18:00 hours), the diurnal variation: (a) morning-evening persisted for the enzymatic defense markers and TBIL (p < 0.001), (b) afternoon-evening persisted for TBIL (p < 0.01) and suppressed for CAT (p > 0.05), and (c) morning-afternoon persisted for GPX (p < 0.001), suppressed for CAT (p > 0.05) and created for TBIL (p < 0.001) with higher values at 08:00 hours. These alteration were suggested to be due to the higher rate of increase recorded in the morning session compared with that in the afternoon and evening. In fact, all variables increased after the training session with respect to pretraining session (p < 0.01) in the 3 TOD with a higher percentage of increase recorded in the morning session compared with the afternoon and the evening (e.g., 25.65 ± 3.87% vs. 16 ± 3.23%; and 13.55 ± 4.09% for CAT; 25.85 ± 4.94% vs. 14.8 ± 1.78% and 13.33 ± 2.04% for GPX). These higher rates of increase during the morning session were also suggested to be in the origin of the delayed (i.e., 48 h) circadian rhythm alteration for the same parameters. Indeed, even after 48 h of recovery period after the weightlifting training sessions, the oxidant/antioxidant balance did not reach the resting diurnal variation and showed higher enzymatic and nonenzymatic antioxidant defenses in the morning compared with the afternoon and with the evening (for CAT, GPX, UA, and TBIL). However, despite this highest percentage of increase generated in the morning compared with the evening training session (34.5 ± 3.3% vs. 22.5 ± 4.1%), no alteration in the diurnal variation of MDA was registered at both 3-min and 48-h posttraining session (i.e., absence of diurnal variation similarly to the resting state). These results suggest a lower sensitivity to the TOD for the lipid peroxidation. The authors in this study (3) suggest that the highest percentage of increase observed in the morning session could be due to the evolution pre-post training session in core temperature (i.e., showed similar response with higher rate of increase during the morning session (4)). This suggestion is in line with previous results of Kanabrocki et al. (37) who showed a significant correlation between increased oxidative damage and increased rate of anabolic-catabolic events (i.e., similarities in the timing of peak values between MDA and parameters relevant to metabolic function such as temperature (33)). Concerning the interaction TOD × physical exercise, significant interactions (p ≤ 0.05) were shown for TBIL and GPX in the study of Ammar et al. (3) indicating that (a) for TBIL, higher values were registered in the morning compared with the afternoon (p < 0.001) only after performing weightlifting task (not observed at pretraining session), and (b) for GPX, higher values were registered in the afternoon compared with the morning (p < 0.01) only at pretraining session (i.e., significant difference was suppressed immediately after the training sessions). These significant interaction (TOD × Training) for TBIL and GPX (i.e., characterized by the creation of higher values of TBIL in the morning compared with afternoon and the suppression of the higher values of GPX in the afternoon compared with the morning after the training sessions) suggest that for these markers, the physical exercise effect dominates the TOD one. This suggestion is in line with previous speculation of Hammouda et al. (28) indicating that the rise in antioxidant markers (i.e., TAS, TBIL, and UA) after exercise did not display a TOD effect (i.e., absence of interaction TOD × before/after RSA test). Until now, given the lack of study in this field, mechanisms responsible for the lack of the TOD effect on antioxidant status after physical task remain unclear.
For the C-reactive protein (CRP), the effect of TOD is still not well established. Indeed, some studies indicated an absence of diurnal variation of CRP concentrations in healthy human subjects (45,47). However, Rudnicka et al. (57) have shown that the predicted maximum for CRP is near 15:00 hours with diurnal amplitude of 17%. Similarly, during physical exercise, contradictory results were shown. Indeed, Rudnicka et al. (57) and Hammouda et al. (31) showed that CRP levels and high-sensitivity CRP (hs-CRP) follow a diurnal pattern with maximal values observed at 15:00 hours in resting state and at 17:00 hours during the Yo-Yo exercise. However, Miles et al. (47) investigated the CRP diurnal pattern during eccentric exercises and showed the absence of significant diurnal variations for both pre-exercise and postexercise values. More recently, Ammar et al. (3) investigated the diurnal variation of CRP in well trained subjects at rest (08:00, 14:00, and 18:00 hours) and during physical exercise (i.e., weightlifting training session) and showed (a) the absence of diurnal variations for this inflammatory marker at resting state and (b) the presence of diurnal variations only from the morning to the afternoon after performing the weightlifting session (i.e., at both 3-min and 48-h postphysical exercises) with higher values in the afternoon. The lack of circadian rhythm for the CRP in the majority of the abovementioned studies, confirm previous suggestions indicating that inflammatory response (e.g., C-reactive level) could be most likely due to generalized muscle damage induced by a high level of oxidative stress but not a matter of chronobiology (65). However, the contradictory results in these studies indicate that further studies are required to well understand the TOD effect in this inflammatory marker.
Effect of Time-of-Day on Core Temperature, Physical Performance, and Fatigue
The results of previous studies concerning the effect of TOD on (a) core temperature, (b) physical performance and fatigue are presented in Tables 1 and 2, respectively.
Souissi et al. (60) showed that body temperature follows a circadian rhythm with an acrophase at 18:00 hours, a batyphase at 06:00 hours, and amplitude of 0.8° C. This fluctuation (Table 1) has been confirmed in trained subjects before and after physical exercises (i.e., RSA, Wingate, and Yo-Yo test) with higher values observed in the afternoon (28–30).
The increase in body temperature during the day can be seen as a passive muscle warm-up increasing nerve conduction velocity and range of motion and decreased muscle viscosity (5,52), which may improve the efficiency of the neuromuscular system. Some studies suggested the possibility of a causal link between changes in core temperature and the diurnal fluctuation in muscular performance (5,16,48). Similarly, recent studies showed a significant correlation between the diurnal fluctuation of biochemical responses to physical exercises and that of core temperature (28,30). The authors suggested that the elevation in body temperature during physical exercise would increase the activity of enzymes such as phosphofructokinase and LDH, which could in turn increase the blood GLC and Lac production. Adding 14:00 hours as a third TOD point, Ammar et al. (4) using a weightlifting training session affirm in part, these suggestions in elite weightlifter. In this context, the authors showed that both resting core temperature and physical performance were higher in the afternoon than in the morning. Comparing the afternoon to the evening session, they showed a higher resting temperature values at 18:00 hours compared to 14:00 hours, whereas performance was better in the afternoon session. Thus, the authors concluded that not only pretraining temperature has an effect on the performance but also the evolution of the temperature during the training session has an effect. In the study of Ammar et al. (4), the authors reported a significant increase of core body temperature of 0.61 ± 0.3%. Joch and Uckert. (36) showed that the increase of 1° C in body temperature generates an increase of the nerve conduction velocity by 2.4 m·s−1 and an increase of 2° C provides greater efficiency in body biochemical reactions. In the muscle level, Mastérovoi (43) showed that this increase in temperature increases the vascularization of muscle groups. These changes of temperature values affect physical performance. In this context, Mohr et al. (48) found that after 5 minutes of play in the second half of a football match, the temperature is always lower by 2° C than the first half, which lead to a better running time during the 30-m sprint (4.57 vs. 4.65 seconds) when the temperature was higher.
Actually, it is well established (Table 2) that short-term maximal performance is better in the afternoon than in the morning (16). In this context, Racinais et al. (52,53) reported that muscle power is higher in the afternoon than in the morning during the 5 × 6-s maximal sprints interspersed by 24-s of passive recovery. In addition, in the study conducted by Hammouda et al. (29), peak power (PP) during the first 2 sprints, total work, and power decrease during the RSA test were significantly higher at 17:00 hours than at 07:00 hours, whereas PP during the last 3 sprints was not influenced by TOD (29). These diurnal fluctuations were also showed during a short-term maximal cycling effort (i.e., Wingate test). Indeed, Lericollais et al. (41) and Souissi et al. (61) reported that PP and mean power (MP) during the Wingate test fluctuate with TOD with acrophases observed at 17:24 ± 00:36 hours and 18:00 ± 01:01 hours (mean ± SD) and amplitudes of 7.6 ± 0.8% and 11.3 ± 1.1%, respectively. They also reported that the fatigue index (FI) was higher in the afternoon than in the morning (41,61). These TOD effects were confirmed in young soccer players (28).
For long-distance exercise, total distance covered during the Yo-Yo test improved significantly from the morning to the afternoon (30) indicating that participants may have greater maximal oxygen uptake values at this TOD point. Ammar et al. (3,4) showed a significant diurnal variation of performance during Olympic weightlifting training session. They reported, also, a raised training load in the 2 Olympic lifts (i.e., the snatch and clean and jerk) in the evening compared with that in the morning (1922.3 ± 235 kg vs. 1792.6 ± 303 kg, respectively). However, adding a third time point of training (i.e., 14:00 hours), they showed a better performance at this TOD compared with the morning and the evening.
Muscle Fatigue and/or Rating of Perceived Exertion
During the RSA test, the rating of perceived exertion (RPE) was significantly higher at 17:00 hours than at 07:00 hours (29). In addition, Racinais et al. (52,53) showed that muscle fatigue, represented by the decrease in power output throughout an RSA test, was greater in the afternoon than in the morning. The same results were showed during 50 maximal voluntary contraction of the knee at 2.09 rad·s−1 (49). Moreover similar diurnal variation of muscle fatigue (i.e., FI: 0.41 ± 0.04 at 07:00 hours vs. 0.49 ± 0.13% at 17:00 hours) has been shown during a 30-s Wingate test (28,60). The above findings (Table 2) suggest that athletes are more fatigable in the afternoon and confirm the diurnal variations of muscle power and fatigue. However, during the Yo-Yo test and Olympic weightlifting training session, a contradictory result was reported. In this context, Hammouda et al. (30) and Reilly et al. (54) showed an inverse diurnal variation of RPE with lower values in the afternoon than in the morning after the Yo-Yo test. Similarly, Ammar et al. (3,4), with an investigation of 3 TOD, showed that RPE responses to a weightlifting training session were lower in the evening than in the morning and more lower in the afternoon compared with the other 2 TOD (FI: 14.3 ± 0.8 vs. 15 ± 0.8 vs 15.3 ± 1.1, respectively for the afternoon, evening, and morning sessions).
The higher performances and muscle fatigue showed in the evening during exercise could be explained by (a) the higher temperature, catecholamine concentrations, levels of leucocyte, CRE, Hcy, Lac, and biological markers of muscle injury, (b) the alteration of ionic movements, and (c) the lower resting antioxidant capacity at this TOD.
In this context, Hammouda et al. (28) suggest a possible link between the biochemical measures and the diurnal fluctuation of short-term maximal performance. Coso et al. (19) concluded that muscle damage is a good predictor of leg muscle fatigue after running. Indeed, the authors indicated that 35% of jump height loss experienced by the triathletes could be explained by blood markers of muscle damage (i.e., myoglobin and CK). Concerning the effect of higher level of WBC and their subpopulations (i.e., MO, NE, LY), Pyne (51) indicated that exercise-induced leukocytosis (i.e., RSA test or 30-s Wingate test) reflects generalized muscle damage and fatigue. For the effect of catecholamine in muscle fatigue and performance, Kanabrocki et al. (38) suggested that higher catecholamine concentrations at a specific time point could possibly affects the diurnal variation in muscle fatigue and performance during short-term maximal exercise. Racinais et al. (52) and Hammouda et al. (28,29) reported that plasma levels of Lac during the RSA or Wingate protocol were significantly higher in the evening, which might explain the higher muscle fatigue and the higher power decrements during the RSA at this TOD. In addition, during intermittent exercise to exhaustion (i.e., Yo-Yo test), the total distance covered during the Yo-Yo test has been shown to be significant correlated to the rate of blood Lac accumulations (40) and to the postexercise Lac levels (30). The above results confirm that Lac level is one of the important underlying mechanism which are in the origin of performance and fatigue fluctuation. Similarly, it has been suggested that increased blood Lac concentrations after exercise in the evening might explain altered RBC deformability and rigidity (70) and seems to make athletes more disposed to inflammation and free radical damage when exercise is done at this TOD.
Creatine levels have also been suggested as an explanation for the performance alteration (i.e., RSA). In this context, Marliss et al. (42) and Bishop and Girard (7) indicated that a strong mobilization of CR (recycled or converted to CR during high-intensity anaerobic exercise) and a high buffering capacity and phosphocreatine replenishment are the most important determinants of the ability to repeat sprints. Concerning its effect in muscle fatigue, Hammouda et al. (29) suggest that the higher levels of CR, Lac, and leukocytosis could be the main cause of fatigue identified in the evening test.
Hcy is an important parameter in the explanation of performance and muscle fatigue given the complexity of its relation with other parameters. In this context, Di Santolo et al. (22) and Joubert and Manore (37) found a positive correlation between Hcy and CR levels. Moreover, Wilson and Lentz (69) support a relationship between high blood levels of Hcy and oxidative stress by activating signal transduction pathways, leading to inflammation and apoptosis. This finding was affirmed by Weiss et al. (67) who concluded that higher Hcy level induces free radical productions in the endothelium, which alters muscle cell membrane permeability during exercise (44).
Diurnal variation in performance and muscle fatigue could be partially, also, induced by alteration of ionic movements (2). Indeed, the increase in magnesium (Mg2+) concentrations decreased the adenosine triphosphate (ATP: direct source of energy) concentrations (8). Moreover, inorganic phosphate (Pi) and potassium (K+) accumulation may decrease the myofibrillar force production and calcium (Ca2+) sensitivity and release from the sarcoplasmic reticulum (23,68). Accordingly, increased Pi is considered to be a major cause of fatigue (68). Moreover, anaerobic glycolysis is of central importance in muscle fatigue. Indeed, the breakdown of GLC to Lac ions and hydrogen (H+) protons may induce early acidosis associated with rapid onset of muscle fatigue (23). Given that the Na+/K+ pump is redox sensitive, Glynn (26) proposed that ionic changes associated with Na+/K+ pump contribute to fatigue.
It should be noted that based on the physiological processes involved in the studied effort (e.g., supramaximal, intermittent, submaximal), different reasons can be given to explain the performance diurnal variation. In fact, Souissi et al. (60) showed that the better aerobic participation in energy production during a 30-s Wingate (maximal anaerobic effort) test in the evening compared with the morning is the main explanation of the better performance in this specific TOD. Hammouda et al. (29) explained the better RSA (intermittent effort) performance in the evening by the greater anaerobic metabolism (i.e., higher mobilization of GLC metabolism) at this TOD (i.e., estimated by the higher levels of Lac, GLC, and CR). Moreover, during long-distance aerobic exercise (24,25,66), the diurnal variations of performance have been reported to be due to (a) the raised anaerobic metabolic activity and muscle fatigue in the evening compared with that in the morning caused by the higher Lac level at this TOD, and (b) the higher oxygen uptake (caused by a slower oxygen uptake kinetics) or (c) to a decrease in maximal oxygen uptake in the morning (55) compared with that in the evening at rest and during submaximal exercise (13,55).
More recently, during Olympic weightlifting exercises, Ammar et al. (3,4) indicated that the higher performance in the evening and the afternoon compared with that in the morning could be explained by the diurnal fluctuation of RPE measures (i.e., lower values after the afternoon and evening sessions) indicating that athletes were less fatigable in the afternoon and the evening. Moreover, in these studies, the worst performance and the higher fatigability recorded during the morning session were suggested to be attributable to (a) the higher increase in the rate of lipid peroxidation and muscle damage and (b) the lower antioxidant resting defense activities at this TOD compared with the 2 other sessions. In this way, Petibois et al. (50) suggested that increased oxidative stress could reflect an imbalance in protein metabolic homeostasis and increased protein catabolism that are associated with exacerbated tissue damage recently considered as an accurate indicator of the diurnal performance fluctuation (4).
The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.
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