Time of day to train has become a “hot” issue in the strength and conditioning field because of the number of athletes being trained, time constraints, limited facility availability, and academic schedules. Thus, more information is needed to gain insights into this complex set of issues, and address the practical question, “What are the effects of training at different times of day?”
Anecdotal observations have suggested a link between athletic performance and time of day because world record–breaking performances in sport were more likely to occur in the early evening (4). Although one may argue that performance in outdoor events is to be superior in the evening because of cooler temperatures, sports performed in climate-controlled conditions, such as swimming, also show comparatively better performances later in the day (6). This time of day effect on performance has been shown in muscle strength and power tasks (17,18), as well as complex motor tasks (5). If the impact that time of day has on performance is not because of external factors such as the environment, internal factors such as circadian hormonal rhythms might play a greater role than previously thought (15,16).
The control center of human circadian rhythms is the suprachiasmatic nuclei of the hypothalamus. These nuclei have receptors for melatonin, which is converted from serotonin and its precursor tryptophan in the pineal gland. When the retina detects light, melatonin secretion is inhibited, and concentrations during the daytime are almost nil. As a result, plasma melatonin concentrations over the course of a day are tied to a relationship with the sleep-wake cycle. Melatonin has been suggested to have both hypnotic and hypothermic properties, and it plays a fundamental role in the onset of sleep (1). Although a lower-body temperature could benefit prolonged aerobic exercise performance, particularly in the heat (14), anaerobic activities with predominantly cognitive and neuromotor components would be most affected by a change in melatonin concentrations because a reduction in body temperature would lessen nerve conduction velocities (10). The optimization of training during “wake” cycles may allow for better “quality” of training, but this wake cycle needs to be better defined with many athletes and fitness enthusiasts alike staying up very late (e.g., after midnight), and in many cases wanting to sleep in late as well (e.g., noontime). However, this is many times obviated by the scheduling of early morning training sessions that need to be completed, and athletes may not be fully awake physiologically (e.g., high melatonin concentrations).
Although interactions between circadian rhythms in short-term or anaerobic physical performance have been previously studied (3,5,7,8,11), less is known about the interactions of between performance with endogenous interactions of melatonin and catecholamines in the context of performance capabilities and endogenous melatonin and catecholamines. Catecholamines have previously been studied in relation to melatonin as the synthesis of hydroxyindole-o-methyltransferase (HIOMT), an enzyme that controls synthesis of melatonin, is suppressed in daylight by noradrenaline. Since catecholamines increase during exercise as part of an adrenergic response to physical stress, there is the potential for catecholamines to offset the normal physiological impact effect of circulating melatonin concentrations. Thus, there is a need for more study on the relationship between these hormones and physical performance at different times in the day. We hypothesized that the adrenal component of the adrenergic response would help to compensate for the higher melatonin concentrations seen in the early morning, which are potentially indicative of athletes who are not yet “physiologically awake” at the time of a training session.
Also, as previously mentioned, an increase in plasma melatonin would likely be detrimental to anaerobic activity and even performance in general because of its hypnotic and hypothermic effects. The concern is that when this occurs, the “quality of training” becomes a primary concern. French et al. (12) have eloquently demonstrated that anticipatory increases in catecholamines are associated with the improvements in force production during a demanding whole body squat protocol (12). Additionally, he showed that dramatic anticipatory (i.e., arousal) elevations in all catecholamines occur at least 10 minutes before this demanding exercise testing the protocol (12). It has also been shown that in a high force “overtraining” scenario, beta-2 receptors in skeletal muscle are down regulated (i.e., desensitized), diminishing their ability to enhance the function and speed of the skeletal muscle's “myosin motor” (13). The importance of epinephrine's influence on Ca++ metabolism and enhanced troponin-C binding may be paramount to epinephrine's direct role in enhancing skeletal muscle performance during exercise. Thus, although greater sympathetic release of norepinephrine may well play an important synergistic role epinephrine role may be vital when increased at the same time as melatonin's elevated diurnal pattern as an “offset mechanism” to maintain typical force/power production in the waking state. Therefore, the purpose of this investigation was to examine the influence of time of day on physical performance in elite track athletes at times when training is often scheduled. Additionally, we sought to better understand the differences in AM and PM concentrations of melatonin and catecholamines. A secondary purpose was to see if the adrenergic arousal response mediated by catecholamines could be implicated as an offset mechanism for melatonin to maintain the negative sedative effects of melatonin.
Experimental Approach to the Problem
A counterbalanced cross-over design was conducted on 2 separate days with 1 week separating the 2 testing sessions at different times of the day. Trained collegiate track athletes were tested at 2 different times of the day to examine the differential effects of melatonin and catecholamine concentrations with respect to physical performance. To test the hypothesis that these factors were different and may be interactive, we measured melatonin and catecholamine concentrations in the early morning (0530 hours) and in the late afternoon (1500 hours) along with exercise performances requiring whole body power or quickness and agility, which demanded different components of motor behavior, and are typical of many training sessions for athletes.
Ten highly trained men who were NCAA Division I track athletes (jumpers and sprinters) volunteered to participate in this investigation. Participants had a mean (±SD) age, height, and body mass of 20.4 (±1.6) years, 185.7 (±9.4) cm, and 77.9 (±8.5) kg, respectively. Each subject was informed of the risks and benefits of the study and provided written informed consent in accordance with the guidelines of the University of Connecticut's Institutional Review Board.
To eliminate what might be considered novel testing stress or learning effect, a familiarization session was conducted. These athletes had participated in early morning workouts, but practice times were typically in the afternoon when the degree of wakefulness may be at its peak. Each subject was familiarized with the testing procedures and equipment before data collection. They were allowed to practice the tests and become familiar with what the testing day would involve. During this time, the participants were also provided with a food activity and sleep log to be filled out for 3 consecutive days before each testing session. We have done this historically to allow for replication of the conditions leading up to the testing, so as to eliminate confounding these major areas of potential confounding variables. Again, using these diaries, subjects were instructed to follow the same pattern of sleep, diet, and activity before each day of testing. Height and body mass were also recorded during this session. Experimental flow of the design is presented in Figure 1.
Testing was conducted on 2 separate days with 1 week separating the 2 testing sessions. Half of the participants started with the AM testing session and half of the participants started with the PM testing session. Participants were asked to drink 750 ml of water before bed and before arrival to assure similar and adequate hydration status of each subject at the time of testing. On the day before testing, caffeine, alcohol, vigorous activity, and naps were prohibited. A phone call reminder was made to every subject at 2200 hours the night preceding testing and at 0430 hours the morning of testing. Each subject fasted for 7 hours before both testing sessions. The AM and PM groups consumed similar meals provided by the investigators preceding the 7-hour fast leading up to testing. The protocol included an initial evaluation of hydration status and core body temperature. Each subject provided a urine sample, which was analyzed immediately to confirm a state of euhydration using a refractometer (Reichert, Lincolnshire, IL, USA) (≤1.019). If a measurement of >1.019 resulted, participants consumed water until they reached or exceeded an acceptable measurement, albeit almost all subjects were euhydrated according to these standards.
Blood Sample Collection
Subjects were seated in a recumbent position for a minimum of 10 minutes before blood draws to stabilize hydrostatic conditions. Resting blood samples were collected from an anticubital vein using venipuncture with a vacutainer and test tubes containing appropriate preservatives. Samples were centrifuged, plasma harvested, and all samples were stored at −80° C until analyses were undertaken. Once blood sampling was completed, subjects prepared for performance testing.
Subjects began laboratory procedures 30 minutes before performance testing. All performance testing sessions were preceded by a 5 minute warm-up of light cycling on a cycle ergometer. Encouragement was provided for each test, similar to what is used during athlete training sessions to create a more relevant atmosphere for testing, so that data could be better generalized to the athletic/fitness populations. Performance testing consisted of an anaerobic testing battery, including countermovement vertical jumps, isometric bench press, and a quick feet reaction test. Except for the quick feet testing, each test was performed 3 times, with the best score used for analysis.
Countermovement vertical jumps were performed on an AMTI force platform (Advanced Mechanical Technology, Inc., Watertown, MA, USA) interfaced with a data acquisition system to measure maximal force (fmax), maximal power (pmax), and maximal jumping height (hmax) using standard biomechanical procedures. Power output and jump height was assessed by performing 3 consecutive maximal countermovement jumps with hands held at the waist.
To properly perform the isometric bench press test, a maximal 10-second push was applied against a fixed bar using a Ballistic Measurement System (Innervations, Lismore, Australia), which incorporates a force plate (FT400; Fitness Technology, Adelaide, Australia) interfaced to an on-line computer. Grip and bar placement was standardized for every subject and used during each trial. The peak maximal force (N) was used for analysis.
The Quick Board (Memphis, TN, USA) was used to assess movement/reaction time. One of the 5 lights located on a box, which was mounted on the wall would illuminate, prompting the subject to react by stepping on a pad that corresponds to the light. Once the correct target was compressed, a new light would illuminate, and this cycle continues until 10 accurate touches were completed. At the conclusion of the trial, time spent and the number of inaccurate touches were recorded. The average from each of the 3 trials was used for comparison.
Plasma melatonin was measured in duplicate using an enzyme immunoassay (GenWay Biotech Inc., San Diego, CA, USA). Free biotinylated antigen was removed and antibody-bound biotinylated antigen was determined by the use of antibiotin alkaline phosphatase. Quantification of unknowns was achieved by comparing the enzymatic activity of unknowns with a response curve prepared using known standards. The lower detection limit of this method was 1.6 pg·ml−1. All samples were assayed in the same run with intraassay differences determined to be ≤4.6 ± 1.2%
Catecholamine analysis was performed in duplicate using high performance liquid chromatography (Bechman Coulter Inc., Fullerton, CA, USA) with electrochemical detection on the ESA Coulochem EC System (ESA Biosciences, Inc., Chelmsford, MA, USA). Samples were prepared, absorbed onto alumina at a pH of 8.6, washed and then eluted with a dilute acid, and subsequently analyzed. An internal standard was included with each extraction to monitor recovery and aid in quantification. Detector parameters for the Coulochem II or III using “A” cells were conditioning cell (+200 mV), Model 5011A cells: E1 (+100 mV) and E2 (−100 mV). Intraassay variances were ≤5 ± 1.3%, and interassay variances were ≤6.1 ± 1.6%.
Statistical analysis was performed using one-way analysis of variance to determine the differences between mean differences between AM and PM measures. When appropriate, to meet the linear statistical assumptions, log10 transformations were used on the data, and transformed data were reanalyzed. Data are presented as mean ± SD. Significance was defined as p ≤ 0.05.
The early morning concentrations of plasma melatonin were significantly greater (p ≤ 0.001) (34.9 ± 22.7 pg·ml−1) than PM values (4.8 ± 3.3 pg·ml−1). The mean resting epinephrine values were significantly (p ≤ 0.05) higher for AM (171.7 ± 33.7 pmol·L−1) than PM (127.6 ± 47.8 pmol·L−1). Mean resting norepinephrine concentrations were not significantly different between AM (1.36 ± 0.45 nmol·L−1) and PM (1.61 ± 0.87 nmol·L−1).
Physical Performance Variables
The quick feet test time was significantly higher (p ≤ 0.05) for AM (5.14 ± 1.06 seconds) than PM (4.39 ± 0.76 seconds). The mean number of mistakes during quick feet trials was also significantly higher (p ≤ 0.05) during AM (2.68 ± 1.61) than PM (2.10 ± 1.08) (Figure 2 and Table 1). Thus, motor behavior related to an integrated signal to movement task was significantly delayed early in the morning.
The overview of the physical performance outcomes from AM to PM can be seen in Table 1. No significant differences were observed in maximal power and force outputs for the countermovement vertical jump or maximal force in the isometric bench press.
The primary findings from this study were that in highly trained collegiate track athletes, power performances are not affected by the time of day, yet quickness and accuracy of movement are worse in the early morning. Importantly, this was observed in the context of melatonin concentrations that were significantly higher in the morning but seemed to be offset by a greater adrenal medullary release of epinephrine. This may be one reason why early morning force/power production capabilities were not compromised, as stimulation of beta-2 receptors may have influenced force production.
A primary question in today's athletic world is “can I schedule athletes to train at 5 or 6 AM in the morning?” Although the answer seems to be yes, one must acknowledge that the potential for a lower degree of physiological wakefulness (i.e., viewed from a melatonin perspective), despite athletes seemingly performing the training as usual. Additionally, arousal through encouragement and cheering did not seem to impact melatonin concentrations but may have influenced the adrenergic response. Nevertheless, it was not great enough to offset the deficit in more complex motor tasks, as suggested by the Quick Board testing. In this study, we had proposed a diurnal variation in the performance of anaerobic activities, yet it only showed up, as noted this was only present in more complex motor task demands. Again, this lack of change in force and power may be explained by an interaction between adrenergic and pineal functions. In contrast to other studies, this study showed that whole body power and upper-body force production were not affected by time of day (17,18). However, performance in a task requiring quickness and accuracy of movement was better in the late afternoon.
Physiological arousal is a normal adrenergic mechanism of preparation for demanding physical performances (12). Although this design did not allow us to directly establish an arousal from baseline levels, observe higher concentrations in the AM were significantly higher than the PM values. This seems to indicate that preperformance adjustments in the adrenal release of epinephrine were higher in the morning. In contrast, we observed no significant differences in norepinephrine concentrations between AM and PM time points, which we have previously noted (12). We were amazed by the specificity of the adrenal medullary response, where epinephrine dominated the release and the lack of other sympathetic influences, such as norepinephrine, a primary neurotransmitter and which has been shown to increase before a demanding workout (12). These data also demonstrate a potentially greater need for epinephrine in the arousal process in the AM, compared with the PM.
The observed interaction between pineal and adrenergic responses is a significant finding: because it may represent a compensatory mechanism that helps to overcome performance decrements resulting from high concentrations of circulating melatonin. This mechanism may have been identified in this highly trained track and field population because these athletes are accustomed to producing an adrenergic response in preparation for rapid force. Importantly, in previous research showing that has showed a diurnal variation in force and power tasks (17), this adrenergic response may not have been observed as result of the population, was not used, and unfortunately, plasma melatonin concentrations were not measured, thus, pointing to the importance of this study to directly examine this issue. It can only be speculated that at 9 AM, it would be likely that melatonin concentrations would have been reduced because of the emergence of daylight, and therefore, no adrenergic compensation would have been required. In this study, early morning performances had the potential to be reduced because of elevated plasma melatonin. However, a slightly elevated adrenergic response, which has not been seen in previous studies, may have been able to counteract such effects. Furthermore, athletes were tested in a simulated typical training environment we created in the laboratory training facility. We had hypothesized that the adrenergic response in the early morning may have negated the performance decrements in strength and power. Nevertheless, quickness and accuracy of movement still showed significant decrements in performances in the early morning were still reduced when compared with performance in the later afternoon. Such data may indicate more complex neurological tasks are either more sensitive to the increased melatonin concentrations, or less sensitive to the level of elevated epinephrine concentrations. Conversely, strength and power tasks involve limiting factors beyond purely neural components such as muscle architecture, fiber type, protein expression, and enzyme levels, which may dilute the impact of melatonin on these measures of performance (9).
Melatonin has been identified as a potential underlying factor that causes changes in performance during the course of the day because of its hypnotic and hypothermic effects (2). This study failed to show differences in strength and power tasks despite significant elevations in plasma melatonin concentrations in the morning. However, we found elevated epinephrine concentrations concomitant to this lack of performance change in force and power and this may explain why a circadian difference in performance was not observed. In comparison with the study by French et al. (12), the magnitude of the epinephrine response was not as high, but the demands of our protocol were considerably less as well. Essentially, we used vertical jumps, drills, and upper-body maximal efforts, compared with the study of French et al. (12), which used a 6 × 10 repetition maximum (RM) at 80% of 1 RM with maximal isometric squat lifts between sets and 2-minute rest periods, with blood lactate concentrations well over 10 mmoL−1. Owing to the fact that we found no changes in catecholamines at our PM time point, despite encouragement and cheering, it seems that such the need for adrenal arousal for the tasks performance could adequately matched the level of epinephrine that was already present before the performance tests, which were about half of the magnitude of comparisons, less than the values observed 10 minutes before the same tests in the AM. Thus, modulation of the sympathetic system is directly related to the “demands” of the workout, because this may well dictate how much the magnitude of adrenal stress itself. Future studies need to examine this same question using more intense workout demands. We might speculate that if the AM demands for epinephrine remain higher than the PM requirements to compensate for elevated melatonin, early morning workouts could contribute to the phenomenon of “adrenal exhaustion.”
The reduction in quickness and accuracy of movement in the early morning seen in this study has been identified previously in strength and power tasks (17–22), as well as complex motor tasks (5). The purpose of this study was to examine the differences in performance that occur at times that track and field athletes typically train (early in the morning or late afternoon), which in the context of a team environment is out of the control of the athletes, whose training is governed by their respective coach. However, with evidence for a weakened physiological state in the morning, the common practice of early morning training should be considered. Furthermore, as well as the fact that while a nadir may occur in the morning, in the late afternoon where performance seems to be superior, athletes may still not be training in their peak physiological state, which appears highly individualized, and may vary between 1200, 1500, and 1800 hours (19,20,22). Although training in a peak physiological state would likely be beneficial, if large individual variations exist, customized training times in a team setting would seem impractical. However, at the very least, it would stand to reason that early morning training and testing should be avoided where possible because this is often the time where athletic performance is at its worst.
To date, sleep experts have categorically indicated a need for 7–8 hours of sleep and there appears to be a needed level of wakefulness to eliminate the influence of melatonin. Obviously, more work is needed from this point to establish continued understanding of these interactions, especially at a time when many athletes are potentially sleep deprived because of schedules and travel. Nevertheless, preexercise modulation of epinephrine and associated adrenergic responses will always naturally occur, but there is concern over the potential overshooting of functional needs, especially if one considers the importance of fine motor skills in many sports. This needs to be established in future research, and it is our hope that this research stimulates a greater interest in this line of study.
The results of this study call into question using early morning practices for conditioning tasks that involve more skilled movements, even when aroused practice conditions are present. Arousal levels mediated by epinephrine did allow maximal strength and power to be maintained in the early AM and PM but were unable to compensate for the level of wakefulness needed to obviate diurnal effects on Quick Board measures. Melatonin is a significant marker of the sleep-wake cycle and may provide further insights into the effects of time of day and wakefulness on athletic performance. Even small differences in melatonin may have dramatic impact on some aspects of performance. Athletes need the recommended amount of sleep (i.e., 7–8 hours), and monitoring sleep patterns seems vital to get athletes into proper phases of the day in to order optimize strength and conditioning workouts.
The authors thank the track and field team and Coach Roy for their support and especially for the help of Coach Joseph Staub (now with the University of Kansas Strength and Conditioning staff). The authors also thank other medical staffs involved with the project and our medical monitor and team physician and Dr. Jeff Anderson for his support. The results of this study do not constitute endorsement of the product by the authors or the NSCA. This study was funded by internal laboratory funds and the authors acknowledge the donation of the Quick Board (Memphis, TN) for use in this research project.
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Keywords:Copyright © 2014 by the National Strength & Conditioning Association.
adrenal medulla; catecholamines; norepinephrine; pineal gland; quickness; strength; isometric bench press; vertical jump