Review: Time of Day Effect on Athletic Performance: An Update

Transcription

1 Journal of Strength and Conditioning Research, 1999, 13(4), National Strength & Conditioning Association Review: Time of Day Effect on Athletic Performance: An Update THOMAS A. CAPPAERT Department of Sports Medicine, University of Charleston, Charleston, West Virginia ABSTRACT Various psychological and physiological functions have been shown to undergo changes relative to the time of the solar day. These variations are known as circadian or diurnal rhythms. These functions exhibit peaks and troughs of maximum and minimum function at specific times of the day. Many components related to athletic performance have been shown to possess these circadian rhythms. A literature search was performed using the databases Medline, SPORT- Discus, and the Cumulative Index to Nursing and Allied Health Literature. Keywords used in the search were circadian rhythm and exercise and diurnal variations and exercise. Articles were used if they were related to athletic performance and/or testing and were published after The articles were then separated into the broad subjects of time of day effects on acute responses to exercise, chronic responses to exercise, and effect of chronotype on exercise. The conclusions reached were that a probable time of day effect is present for the following conditions: isotonic and isokinetic strength measures, anaerobic power and capacity, and body temperature and exercise response. A more equivocal relationship was found between time of day and endurance training, ratings of perceived exertion, chronotype and exercise, arm exercise, and self-paced exercise. Key Words: circadian rhythm, exercise, diurnal variations, exercise testing, body temperature Reference Data: Cappaert, T.A. Time of day effect on athletic performance: An update. J. Strength Cond. Res. 13(4): Introduction The idea of a variation throughout the solar day of physiological and psychological variables is not recent. Daily variation in body temperature was first reported in 1778 (15). These time-dependent variances are known as circadian rhythms. These psychophysiological functions exhibit maximum and minimum phases throughout the day. Many of these can have an effect on sports performance. It is important, therefore, for coaches and athletes to be aware of how the time of day will affect various components of physical performance. The hallmark of a good training program is the identification and maximum improvement of the components necessary to excel at a particular athletic endeavor, be they physical or mental. If one of these components has a time of day variance, then it behooves the coach and/or athlete to schedule the activity at the time that the particular component will be at its maximum effectiveness. This has applications in planning training sessions, testing sessions, and competition times. Previous research (20, 26, 29) has addressed the time of day effects on basic psychophysiological responses at rest and exercise. Many of these responses are related to athletic performance. Recent research has begun to study some of these basic responses as they relate to more specific aspects of physical/athletic performance. The purposes of this article are to review the recent literature as it pertains to circadian rhythms in variables of athletic performance, to reach conclusions concerning this research, and to devise recommendations to apply these findings to practical athletic performance and testing. Literature Review Several psychophysiological factors in sports performance have been identified as following a circadian rhythm (Table 1). The research conducted before 1985 can be obtained in detail from other reviews (20, 26, 29). Much of the research conducted before 1985 was performed to measure the time of day effect on more basic physiological functions, such as body temperature and heart rate at rest and oxygen consumption at various intensity levels of exercise. Other performance parameters that have been found to possess a circadian rhythm include stroke volume, cardiac output, blood pressure, vascular blood flow, metabolic rate, sweat rate, and trunk flexibility. Table 1 summarizes the broad categories of physical performance that have been found to possess a circadian rhythm. The table includes the time of day found to correspond to peak function of each factor. This review will attempt to 412

2 Time of Day and Athletic Performance 413 Table 1. Psychophysiological variables of athletic performance affected by circadian rhythm.* Variable Sensory motor: simple reaction time Psychomotor: hand/eye coordination Sensory perceptual: pain threshold Cognitive: information processing Neuromuscular: strength Psychological Affective: mood Psychophysiological: arousal Time of peak effect (h) Cardiovascular: heart rate Metabolic: body temperature, resting oxygen consumption Aerobic capacity: maximal oxygen consumption * Adapted with permission from Winget et al. (29). supplement previous work and review research published since Chronic Exercise Response and Time of Day Training programs are designed to elicit the maximum improvement in the fitness variables that are vital to a particular athletic performance. The chronic physiological adaptations seen after endurance training are well known. The time of day that the training takes place has been given attention only recently as a possible determinant of the extent of the training adaptations. Two studies have addressed this possibility and, from their results, it appears that there is a time of day effect on the improvements found after undertaking an endurance training program. Hill et al. (9) studied 7 men and 20 women with average ages of 26 and 24 years, respectively, and with moderate levels of activity. The subjects performed maximal cycle ergometer tests at 2 times: hours and hours. Subjects were then assigned randomly to an AM or PM exercise group or a control group. The 2 exercise groups performed a 6- week endurance exercise training program consisting of interval and continuous cycling and/or running workouts. Temperature and humidity were controlled for all testing and training sessions. The interval work was performed at % of V O2 max, and continuous work was performed at the highest sustainable intensity for minutes. The training program produced a significant training effect with decreased exercise heart rate, increased V O2 max, and increased V O2 at the ventilatory threshold. Also improved were minute ventilation, rating of perceived exertion (RPE), and a decreased heart rate at submaximal exercise levels. Performance times were increased for maximal exercise as well. There were, however, no significant time of day differences in adaptation of heart rate, minute ventilation, V O2, or RPE during submaximal or maximal exercise. No circadian variation was found for performance time during maximal exercise. It was discovered that subjects who trained in the morning had a higher minute ventilation during morning exercise, and subjects who trained in the afternoon had a higher minute ventilation during afternoon exercise. No circadian specificity was found for V O2 max, however. Because of this ventilation specificity, the authors suggested that training should coincide with the time of day at which performance is scheduled (9). A more recent study by Torii et al. (28) also examined the time of day effect on response to an aerobic training program. The investigators studied 12 sedentary men with a mean age of 30 years. Heart rate and blood lactate concentrations were measured before and after a 4-week training program using a 30-minute exercise bout at 60% of predetermined V O2 max. The tests were performed at the same time that the subjects were to train. The subjects were assigned randomly to a morning ( hours), early afternoon ( hours), or evening ( hours) exercise group. The subjects then trained 4 days per week at durations and intensities identical to the pre- and posttest conditions. These included controlled environmental conditions. This training continued for 4 weeks. After the training, there were no significant differences found among the 3 exercise groups for resting heart rate. Significantly different improvements were found in V O2 max, recovery heart rate, and blood lactate response for the afternoon exercise group, however. The investigators concluded that training of this type, intensity, and duration appears to have a more pronounced training effect in the afternoon as compared to the morning or the evening (28). From these results, it can be surmised that endurance training may have a time-specific adaptation, especially the training effects for minute ventilation. However, the paucity of research in this specific area precludes a definitive conclusion. These results may lend credence to the idea that training should take place at the same time that optimal performance is required. This view of training time specificity concurs with the conclusions of Winget et al. (29), in that training or competition should take place at the times that performance is expected to be at its peak, generally in the afternoon. Several possible shortcomings in these 2 studies are apparent, however. The afternoon-specific improvements in V O2 max, recovery heart rate, and blood lactate response as seen in the study by Torii et al. (28) seem to suggest that endurance training would best be undertaken in the afternoon. However, this study used a relatively short 4-week training program of relatively low to moderate intensity with untrained subjects. A longer, more intense program with trained subjects may yield different results. The study by Hill

3 414 Cappaert et al. (9) was also conducted with a relatively short training program, but with an adequate intensity level. An interesting alternative study design would be a within-subjects design, with the 2 groups spending equal periods training in the afternoon and then the morning, with a period of inactivity in between to allow the subjects to return to baseline fitness levels. This would allow the subjects to act as their own controls and would decrease between-subject error. The dependent variables used appear to be appropriate to gauge chronic training adaptations, but a wider variety of exercise times throughout the day may clarify the true optimum time to train. Additional research areas could include a time of day effect on prolonged strength training, training programs using trained subjects, and training of combinations of variables, such as aerobic endurance and muscular strength. Acute Exercise Response and Time of Day Circadian Rhythms and Perceived Exertion. Ratings of perceived exertion are often used to monitor exercise intensity. A time of day effect on RPE may alter the perception of exertion, thus causing an over- or underestimation of actual exercise intensity. This could directly affect the acute and/or chronic responses to the exercise. The following study presents some evidence that perceived exertion is affected by the time of day. A study conducted by Hill et al. (10) attempted to identify a time of day effect on RPEs during exercise above and below the ventilatory threshold. The subjects included 8 men with a mean age of 25.6 years and 24 women with a mean age of 23.6 years. Two maximal graded exercise tests were performed on the same day. The tests were conducted between hours and hours. The RPE was measured using the 15-point Borg scale every minute during the test. V O2 max and ventilatory threshold were determined from the test data. The results showed no significant differences in maximum heart rate, ventilatory threshold, and RPE at the ventilatory threshold between the AM and PM exercise sessions. V O2 max was significantly lower as measured during the AM sessions compared with the PM sessions. The RPE at intensities below the ventilatory threshold showed no significant differences between the AM and PM exercise sessions. However, RPE measures at intensities above the ventilatory threshold proved to be significantly lower in the AM sessions than in the PM exercise sessions. The investigators attributed this difference to the generally lower ventilatory demand present in the morning. The authors also concluded that the effect of time of day should be considered during practical applications of RPE during exercise (10). The results of this study seem to suggest that at cycling intensities above the ventilatory threshold, there is a time of day difference for RPE. This effect did not continue into maximal exercise. These results are in disagreement with 2 previous studies (4, 16) that showed a significant difference in RPE during maximal exercise, with the maximal ratings occurring in the afternoon or evening. If the RPE difference above the ventilatory threshold in the AM is due to lower ventilatory demands in the morning, then theoretically this lower ventilatory demand should make maximal exercise be perceived as less strenuous as well. A possible shortcoming of this research is the ordering effect of performing both tests on the same day without random assignment. Other physiological factors such as heart rate and body temperature could contribute to the differences in the perceived exertion, and these may need to be addressed in conjunction with perceived exertion to attempt to establish a causal relationship. Additional research is necessary in this interesting aspect of exercise and time of day, especially to mitigate the conflicting results of the studies cited and to clarify the possible physiological factors that could contribute to differences in perceived exertion. Circadian Rhythms and Strength Measures. Strength testing or measurement is often used to determine baseline or progress data for athletes in conjunction with strength or rehabilitation programs. Recent attempts have been made to identify a time of day effect for various muscular strength measures. These studies present convincing evidence of a time of day effect on isotonic and isokinetic strength measures of the upper and lower extremity. Lundeen et al. (18) studied 21 women and 25 men of college age. Measures of maximal strength, maximal speed of contraction, and muscle power of the quadriceps were attained using an isotonic ergometer. Handgrip strength was measured using a hand dynamometer. Manual dexterity was measured using a timed maze task. A score was determined by using time to completion and number of faults to produce a dexterity index. Measures were made periodically over a 24-hour span. Significant differences in maximal quadriceps strength, speed of contraction, and muscle power were found in the afternoon measures compared with the morning measures. A statistical difference was also found between genders, with the peak for women occurring about 4 hours earlier in the day compared with the men. Grip strength also showed a time of day difference with the peak in the afternoon. No attendant gender differences were found. Manual dexterity measures, however, exhibited a morning peak with a significant gender difference. The peak time for men was approximately 1.5 hours earlier than the peak time for women. Time of day differences in endocrine function, as measured by plasma cortisol and -endorphin as well as levels of catecholamines in the urine, mirrored the differences in muscular strength. According to the investigators, this factor may suggest a biochemical reference rather than a

4 Time of Day and Athletic Performance 415 clockwise reference to performance differences. The investigators also concluded that time of day should be considered when optimal performance of these measures is a factor (18). Another study that examined leg strength and time of day differences was undertaken by Wyse et al. (30). This study used isokinetic strength measures of knee flexion and extension. Nine college-age male athletes performed isokinetic exercise at hours, hours, and hours on 3 separate days. Four maximal voluntary contractions were performed at velocities of 60 s 1 and 180 s 1. Peak torques of the knee extension and flexion movements were determined. A peak torque ratio between knee flexion and extension was also determined. Significant time of day differences were found for all measures between the session at hours and the 2 earlier sessions at both measured velocities. The investigators concluded that the optimum time for maximal isokinetic leg strength measures occurred between hours. The conclusion was also reached that practical comparison of maximal isokinetic leg strength should be done only between data obtained at similar times of the day, with a margin of 30 minutes (30). The results of these studies present moderately convincing evidence of a time of day effect for isotonic and isokinetic strength measures. These results would seem to be in agreement with studies showing an afternoon peak in elbow flexion strength (5) and bilateral handgrip strength (6, 16, 17). Possible shortcomings of the research include small sample sizes; performance of all testing on the same day, possibly resulting in an ordering or learning effect (18); and nonfunctional exercise patterns and speeds. It is difficult to directly relate the dependent variables studied to actual sports skill performance, and this could be addressed in future research. Interesting variations of these studies for future research could include using different muscle groups, studying the possibility of a time of day variance on training effects of a strength training program, and using more functional and/or dynamic movements to measure strength. Circadian Rhythms and Anaerobic Exercise. A relatively new aspect of physiological variables being studied in terms of circadian rhythmicity is short-term, highintensity exercise. Anaerobic power or capacity is applicable to many sports and sports skills, and maximizing this ability is a top priority for athletes. The studies to be discussed show a pattern of a time of day effect during anaerobic exercise using cycle ergometry and some functional muscle power tests. Hill and Smith (13) studied 6 college-age men who each performed a modified Wingate anaerobic cycle test. These tests were performed at 0300, 0900, 1500, and 2100 hours, with tests ordered randomly 5 7 days apart. Work resistance was preset at 5.5 kg and was the same for all subjects. Peak power, as defined by the highest power output in a given 5-second period, and anaerobic capacity, defined by the total external work during the 30-second period, were measured. Significant differences were found in peak power between the and 2100-hour means. Significant differences were also found in anaerobic capacity between the and 1500-hour means. The results suggested a peaking pattern for power output and anaerobic capacity as measured by the modified Wingate test, with slow increases from early morning to a peak in the time span of hours. The investigators concluded that time of day should be considered when performing anaerobic tests measuring peak power and anaerobic capacity (13). Another study by Hill et al. (7) studied intense short-term exercise using a maximal cycle test to exhaustion. Twenty-five college-age subjects (9 women and 16 men) were recruited to perform 2 maximal cycle ergometer tests. Resistance was constant and was preset at 5 W kg 1 for women and 6 W kg 1 for men. The subjects were assigned in a random order to exercise in a morning session ( hours) and an afternoon session ( hours). Time to exhaustion was measured for each testing session. The results revealed a statistically significant difference, with afternoon-session exhaustion times greater than those of the morning session. There was no interaction between gender and the time of day, however. The investigators concluded that time of day should be considered when using repeated, high-intensity tests (7). Hill et al. (8) again studied all-out cycle exercise to determine a possible time of day effect on total work, time to exhaustion, anaerobic work, aerobic work, and aerobic power. Six women and 8 men of college age exercised at a maximal rate to exhaustion. A constant, preset workload of 5 W kg 1 for women and 6 W kg 1 for men was used for the tests. Two identical, randomly ordered work sessions were conducted at and hours. Total work, time to exhaustion, aerobic power, anaerobic work, and aerobic work were all significantly greater in the afternoon sessions than in the morning sessions. No significant differences were found relative to gender and time of day. The investigators concluded that time of day appears to affect performance of short-term, high-intensity, constant power cycling exercise. This relationship is independent of gender (8). A time of day effect on anaerobic power was investigated by Reilly and Down (23) in a more functional manner, using the Margaria stair-run test, a standing broad-jump test, and a Wingate anaerobic cycle test. The subjects were 12 college-age men who performed the tests at 6 separate times (0200, 0600, 1000, 1400, 1800, and 2200 hours) on different days. Power output as measured by the Margaria stair-run test and distance attained in the broad-jump test

5 416 Cappaert showed significant time of day differences, with peaks occurring at hours. Peak and mean power as measured by the Wingate cycle test did not, however, present a significant time of day difference. The authors believed that the error inherent in dynamic ergometer tests and the nonrandom ordering of the tests on each occasion may have overridden any circadian rhythm in peak and mean power as measured by the Wingate test. The authors concluded that a time of day effect was present when measuring anaerobic power using the standing broad-jump and Margaria stair-run tests (23). The most recent investigation performed on the time of day effect on anaerobic power studied 13 female college-age physical education students. The Wingate anaerobic cycle test was performed at 4 times of the day 0300, 0900, 1500, and 2100 hours all on the same day. Peak and mean power were measured. A statistically significant difference was found for peak power, with the highest mean occurring at 1500 hours as compared with 0300 and 0900 hours. Significance was found for mean power as well, with the highest means occurring at 1500 and 2100 hours as compared with 0300 hours. The author noted that the variations in power outputs correlated with a similar variation in body temperature. The author did not believe that a significant ordering effect was present because of the fitness level of the subjects and the fact that power measures improved throughout the day. This fact led the author to rule out any residual fatigue effects. The author concluded that time of day should be accounted for when using supramaximal exercise in studies, training programs, or competitive events (19). This recent evidence appears to solidify the presence of a circadian rhythm in anaerobic power and capacity, with a peak in the afternoon that is independent of gender. It is still unclear from the present data whether these findings will carry over into more functional, performance-specific activities such as sprinting, throwing, or repeated jumping. The findings of a time of day effect for the standing broad jump and Margaria stair run appear to bolster that idea, but this aspect needs more evidence. The use of peak and mean power measurements, maximal exercise time to exhaustion, and the more functional power tests used by Reilly and Down (23) to quantify anaerobic power is appropriate to begin establishing causal relationships with time of day effects. Three significant shortcomings are present in the research concerning anaerobic measures. The first is the limited number of data points taken during the solar cycle, in some cases only 2. This may not allow one to discover a true circadian effect and the peak time of performance. The second shortcoming is the testing of all the data points on the same day. This design may lead to an ordering or learning effect. The third limitation is the small sample size used in the majority of these studies. Future research topics in this area may include a possible time of day effect on repeated bouts of anaerobic exercise, a time of day effect on anaerobic exercise using different exercise modalities such as running or circuit training, and whether the time of day effects associated with improved acute responses to exercise will lead to improved chronic adaptations compared with a time of day with lesser acute responses. Circadian Rhythms and Arm Exercise. Most of the studies probing the question of circadian rhythms in exercise response have used the larger muscle groups of the legs. Upper-body training responses would be important for several sports, such as throwing sports and swimming. There appears to be a possibility of a time of day effect in the performance of maximal arm exercise. A study by Cable and Reilly (3) looked for a time of day effect on maximal arm exercise. Twelve collegeage men performed a maximal graded arm ergometer test to exhaustion. This test was performed at 0200, 0600, 1000, 1400, 1800, and 2200 hours on different days, with random assignment to times. Resting measures of heart rate, body temperature, and minute ventilation were found to be significantly higher in the afternoon for all subjects. During exercise at submaximal intensities (light and moderate), an afternoon peak was also noted for heart rate and body temperature. An afternoon peak was also noted for V O2 and minute ventilation at light intensities, but not at moderate intensities. No time of day effect was noted for RPE at light and moderate intensities, with the highest RPE mean occurring at 0535 hours. This peak coincided with the lowest body temperature mean. During maximal exercise, time to exhaustion showed statistical significance with a peak at 1650 hours, which coincided with the highest body temperature mean. An afternoon (1653 hours) peak of heart rate during maximal exercise occurred as well. Relative V O2 max, V CO2, respiratory exchange ratio, minute ventilation, and V E/V O2 ratio all had significant afternoon peaks. The only recovery variable that presented a significant difference was that of minute ventilation, with a peak at 1504 hours for the fourth minute of recovery. The authors concluded that the muscular efficiency of arm exercise is independent of time of day relative to a constant respiratory exchange ratio. An additional conclusion reached was that the strong association between peaks in body temperature and time to exhaustion could be an important factor in optimal performance of maximal arm ergometry (3). The results of this study give some early indication of a time of day effect on maximal arm exercise that mirrors the effect that is seen with lower-body exercise. These data appear to concur with the results of a previous study (1) that suggested a circadian rhythm for arm exercise as performed during all-out swimming. The probable relationship between peak body

6 Time of Day and Athletic Performance 417 temperature and peak exercise performance appears to occur for upper- and lower-body maximal exercise. The limited research, however, makes a definite conclusion difficult. Additional research using maximal arm exercise is necessary to substantiate these results. Circadian Rhythms of Body Temperature During Exercise. A significant limiting effect of endurance exercise in the heat is the body s ability to maintain satisfactory thermoregulation and prevent overheating. The circadian rhythm of body temperature has been demonstrated to occur in the afternoon (29). The effect of an elevated body temperature could have negative effects on the thermoregulation of the body in the heat. Two studies have shown that exercise or exposure to heat does not affect the circadian rhythm of body temperature. This will have consequences on the body s ability to adapt to increased temperature during exercise. Reilly and Brooks (22) had 15 men with a mean age of 26.2 years perform a maximal cycle ergometer exercise test. This test was performed at 0200, 0600, 1000, 1400, 1800, and 2200 hours on separate days. Laboratory temperature was controlled for all testing sessions. Mean body temperature and mean skin temperatures were measured at light, medium, and maximal exercise intensities. Time to exhaustion during maximal exercise was also recorded. Time to exhaustion was found to be independent of time of day. Significant time of day differences were found for mean skin and body temperatures at all exercise intensities and into the recovery period. These measures all peaked in the afternoon and rose in harmony with exercise intensity. The authors concluded that these results are in accordance with the phases of body and skin temperature increases seen at rest (22). Hill et al. (12) chose to study the effect of circadian rhythms in body temperature on oxygen uptake during exercise. Twenty-seven subjects performed a maximal graded cycle ergometer exercise test between hours and between hours on the same day. Conditions of temperature and humidity were controlled for all sessions. Significant differences were found for V O2 at all work rates, with the highest values occurring in the PM exercise session. The V O2 / work rate slope was the same for the AM and PM exercise sessions. A significant correlation was also seen between the afternoon increases in body temperature and the afternoon increases in the V O2 /work rate slope above the ventilatory threshold. This accounted for 10% of the difference. A significant correlation was also seen between the afternoon increases in body temperature and in oxygen consumption below the ventilatory threshold. This variation in temperature accounted for 20% of the difference in afternoon oxygen consumption. The authors concluded that the higher body temperatures increased demands on the respiratory and thermoregulatory systems at a given intensity of exercise, thus producing a higher V O2 (12). Zahorska-Markiewicz et al. (31) studied the effects of heat exposure on the circadian rhythm of body temperature during exercise. Ten men with a mean age of 28.5 years were tested at randomly assigned times of 0600, 1200, 1800, and 2400 hours on different days. Each subject was exposed to conditions that consisted of a room temperature of 42 C and 60% relative humidity for 50 minutes at rest. The subjects then exercised for 10 minutes on a cycle ergometer at 50% of their predetermined V O2 max. Heart rate during the heat exposure was significantly higher at 1200 and 1800 hours compared with 2400 hours. The heart rate mean during the exercise in the heat was significantly higher at 0600 hours than at 2400 hours and significantly higher at 1200 hours than at 2400 hours. Metabolic rate (W) during rest was significantly higher at 2400 hours than at 0600 hours. The metabolic rate was also higher during heat exposure at 2400 hours compared with 0600 hours and higher during exercise at 1200 hours compared with 1800 hours. Rectal temperature was significantly higher at 1200 and 1800 hours compared with 0600 hours at rest, during the heat exposure, and during the exercise bout. The authors concluded that body temperature with heat exposure exhibited a circadian rhythm independent of skin temperature changes, whereas heart rate during exercise in the heat and with heat exposure did not (31). These results provide evidence that the circadian rhythm of resting body temperature continues during exercise of moderate to high intensity. This relationship may also be causally related to the rhythm of exercise performance. The study of Zahorska-Markiewicz et al. (31) did not find a circadian rhythm for heart rate during exercise. The effect of the heat exposure and the low intensity of the exercise bout may have overridden the circadian rhythm of heart rate during exercise. These data give further strength to the idea that exercise in the heat must be monitored closely. If we can expect a rise in body temperature due to a circadian rhythm before exercise even begins, then this should be accounted for with increased vigilance to prevent overheating. The dependent variables used in these studies appear to be appropriate indicators of physiological stress during exercise, but it appears that more work could be done to clarify the effects of heat, time of day, and heart rate. Further research should be conducted using longer steady-state exercise bouts to better address heat-tolerance thresholds relative to time of day. This issue could also be combined with the monitoring of perceived exertion and perhaps a more detailed analysis of cardiovascular function, such as cardiac output measures. Circadian Rhythms and Self-Paced Exercise. Prolonged endurance exercise is reliant on the ability of an athlete to select a sustainable pace or intensity level. The ability to correctly pace effort or the perception of that effort may be affected by time of day.

7 418 Cappaert Table 2. Studies on the effect of time of day and exercise performance. Reference Dependent variable exhibiting time-of-day effect Exercise mode Time of day coinciding with peak effect (h) Hill et al. (10) Morning, trained V E Cycling Afternoon, trained V E Torii et al. (28) Hill et al. (9) Lundeen et al. (18) Wyse et al. (30) Hill and Smith (13) V O2 max improvements Recovery heart rate improvements Blood lactate response Rating of perceived exertion above ventilatory threshold Maximal quadricep strength Maximal quadricep contraction speed Quadricep muscle power Grip strength Manual dexterity Plasma cortisol levels Beta endorphin levels Catecholamine levels Isokinetic peak torque for knee extension and flexion Cycling Cycling Isotonic ergometer Hand dynamometer Isokinetic dynamometer Isokinetic peak torque ratio Peak power output Anaerobic capacity Wingate cycle test Hill et al. (7) Maximal exercise time Cycle Hill et al. (8) Maximal exercise time Anaerobic work Aerobic work Aerobic power Total work Cycle Reilly and Down (23) Melhim (19) Power output Broad jump distance Peak power Mean power Cable and Reilly (3) Maximal arm exercise Arm ergometer Time to exhaustion Heart rate V O2 max Respiratory Exchange Rate (RER) V E V E /V O2 ratio Margaria stair run test Wingate cyle test Reilly and Brooks (22) Mean body temperature Cycle 1709 Mean skin temperature 1651 Hill et al. (12) Zahorska-Markiewicz et al. (31) V O2 max V O2 submax Exercise at 50% V O2 max Heart rate Metabolic rate Rectal temperature Cycle Cycle Reilly and Garrett (24) Self-paced exercise Cycle 0 20 minute power output minute power output

8 Time of Day and Athletic Performance 419 Table 2. Continued. Reference Dependent variable exhibiting time-of-day effect Exercise mode Time of day coinciding with peak effect (h) Hill et al. (11) Morning types Cycle Submaximal RPE Submaximal V O2 Maximum heart rate Evening types Submaximal V O2 Maximum heart rate V O2 max Burgoon et al. (2) Time to exhaustion Treadmill test A recent study attempted to compare physiological responses to prolonged exercise in the morning and evening. Reilly and Garrett (24) recruited 7 male collegelevel soccer athletes to perform a 60-minute maximal cycle ergometer test. The subjects were to work against a resistance preset to 5% of their body weight, were to exercise as hard as they could over the duration of the test, and could vary pedal frequency as they wanted. Subjects were classified as morning, evening, or intermediate types based upon answers to a questionnaire. The subjects then performed this exercise bout at 0830 and 1730 hours with at least 72 hours between tests. Power output and rectal temperature were measured every 10 minutes. Pre-exercise rectal temperature was significantly lower in the morning, as was postexercise temperature. Mean power output did not have a significant interaction with time of day. The self-set exercise pacing did show significant differences. Significantly higher power output values were noted for the first 20 minutes of the evening exercise bout compared with the morning exercise bout. Significantly higher power output values for the middle and last 20 minutes of the morning exercise session compared with the evening session were also noted. The authors concluded that these results might suggest a reluctance to exercise intensely in the morning until the subject is more adequately warmed up. The authors also concluded that this investigation suggests the likely beneficial effects of a warm-up before high-intensity exercise of minutes in duration. This effect would likely be more pronounced in the morning (24). The fact that the afternoon body temperature mean was higher than the morning mean suggests a greater stress on the thermoregulatory systems of the subjects during the afternoon exercise sessions. This may have accounted for the decreased performance of the afternoon exercise group in the latter stages of the exercise bout. This result is in disagreement with a previous study (21) showing an afternoon peak for ergometer performance to exhaustion. The telling difference is in the ability to self-pace the exercise bout, and this probably accounts for the difference in the results. Another factor that could be influencing the results is the perceived exertion of the exercise. If the first 20 minutes of morning exercise feels harder, then the self-set pace may be lower. An additional factor that could be studied in relation to the self-paced exercise is the cardiac response (e.g., heart rate, cardiac output). Two additional areas for research include whether there are thermoregulatory differences in morning exercise for exercise bouts of a longer duration and whether prolonged exercise can be maintained at a self-set pace longer in the morning than in the evening. Effects of Chronotype The idea of chronotypes, or time of day preferences that can be expressed as morning and evening types, is not recent (14). The hypothesis has been developed that a person s preference for morning or evening activities may also have an impact on their acute response to exercise. The research does not seem to support this assertion, however. Hill et al. (11) recruited 8 men with a mean age of 26 years and 24 women with a mean age of 24 years. The subjects were then classified as a morning type (n 14), an evening type (n 11), or neither (n 7) based upon responses to a questionnaire. The subjects then performed 2 maximal progressive cycle ergometer tests: 1 test between 0600 and 0830 hours and 1 between 1530 and 1800 hours on the same day. No significant differences were found between the groups for heart rate, V O2, or RPE at submaximal levels. A significant difference in RPE for the morning types showed that the submaximal level appeared more difficult during the AM test. A significant time of day difference was present for all groups in V O2 at submaximal levels, with the PM values being higher. No significant time of day differences were present for heart rate, V O2, work rate, or RPE at the ventilatory threshold. There was a significant difference in maxi-

9 420 Cappaert mum heart rate between groups, with the morning types having a higher heart rate in the AM and the evening types having a higher maximum heart rate in the PM. The evening types also had a significantly higher V O2 maxinthepm compared with the morning types. However, no differences were apparent in maximum performance time. The authors concluded that exercise response is a function of time of day, that morning or evening type should be considered when testing V O2 max at various times of the day, and that resting and submaximal exercise variables are the same for morning and evening types (11). A similar study by Burgoon et al. (2) tested 26 men of college age who had been typed as morning (n 9), evening (n 6), or intermediate (n 11) people. These subjects performed a maximal graded treadmill test at hours and hours in a random order 48 hours apart. Significant differences were found for time of day and time to exhaustion, with higher values at the PM testing time. No interaction between time of day and V O2 max was discovered. No between-group differences were found for V O2 max or time to exhaustion. The authors concluded that V O2 max, as measured by maximal treadmill test, is the same independent of morning or evening type. Time of day should be considered when using maximal treadmill exercise (2). These mixed results do not appear to substantiate the anecdotal evidence of improved performance by athletes depending on their preferred exercise period. A previous study (25) showed that higher performing morning-sport athletes had higher morningness scores than high performing afternoon-sport athletes. This appears to make the morningness/eveningness issue more of an individual difference than a general assumption. This relationship needs further clarification and may need to be tested as it relates to performing a specific sport skill and performance in a competitive environment. The questionnaire used (14) may need to be examined as well. This self-reporting instrument may have some flaws in that there is no way to relate these personal preferences to physiological responses and thus to performance as measured in these studies. Another area of research that should be addressed is time of day preference and its relationship to submaximal and maximal exercise. In addition, the mechanisms for physiological variations as they relate to morning or evening preference need to be explored. An additional consideration related to study design for all studies addressing the issue of the effect of time of day on exercise response is the relation between the amount and timing of sleep and the performance of exercise. The influence of how long ago the person woke up compared with initiation of exercise may also be a factor involved in the acute and chronic responses to exercise. That association is not easily examined based upon the results of the research reviewed here, but it appears to be an issue that should be examined. This could be considered a future research direction. A summary of the results of the studies reviewed here can be found in Table 2. Practical Applications Based upon the results of previously reviewed investigations and of the studies reviewed in this article, a time of day effect on exercise is a factor that must be taken into account. This would have practical applications in planning training sessions, testing sessions, experimental investigations, and competitive events. Optimal performance is the paramount objective of athletic endeavor, and adjusting for time of day differences appears to be an important factor in attaining that optimal performance. Based upon this review, the following guidelines or recommendations can be set forth: (a) Endurance training should probably be conducted at the same time of day that performance is scheduled. This effect seems independent of gender. The question of whether morning or afternoon endurance training is more beneficial is still unresolved. The answer would have implications for designing training programs to enhance aerobic capacity. (b) When using RPEs to monitor exercise intensity above the ventilatory threshold, time of day should be considered in interpreting ratings. This factor would have implications when monitoring intensity during training or testing. (c) Handgrip strength measured isotonically and knee flexion/extension measured isotonically or isokinetically should probably be performed in the afternoon. The earlier leg-strength peak of women should also be considered. When comparing separate isokinetic test results, the testing times should be within 30 minutes of one another. This factor would have the most impact during testing to attain baseline or progress measurements for an athlete. (d) Performance of short-term, high-intensity exercise should probably be scheduled in the afternoon to realize maximum performance. This effect is independent of gender. This factor has implications for testing, the training of anaerobic components of fitness, and scheduling of competitions. (e) Morning and evening types may need to be considered when using maximal exercise or measuring V O2 max. The use of the Horne-Ostberg questionnaire (14) or the questionnaire suggested by Smith et al. (27) can help to establish morning or evening preference, and then testing, workouts, training programs, or competitions can be scheduled accordingly. (f) Limited evidence shows that maximal arm exercise probably should be performed in the afternoon

10 Time of Day and Athletic Performance 421 to maximize performance. This may have implications for testing, training, and scheduling competitions. (g) Increased body temperature during afternoon exercise should probably be accounted for when scheduling maximal exercise. This effect may limit performance because of increased thermoregulatory demand compared with morning exercise. This factor would have implications in the scheduling of testing, training, and competition, especially prolonged endurance events. (h) Adequate warm-up should be performed before morning exercise to ensure proper physiological readiness for intense exercise. This factor will have implications for testing, training, and competitions. (i) Training and competition should be scheduled at generally the same time of the day. If this is impractical, the athlete may be able to adjust the sleep/ wake cycle so that the performance time will coincide with the adjusted peak times for a particular rhythm. This is in agreement with the recommendations of Winget et al. (29). (j) Peak times for certain performance variables are generally subject to individual differences and will need experimentation by the athlete to find the optimum performance time for these variables. Note: Thomas A. Cappaert is now at Welltrack, 2051 W. Central Ave., Toledo, OH References 1. BAXTER, C.,AND T. REILLY. Influence of time of day on all out swimming. Br. J. Sports Med. 17: BURGOON, P.W., G.J. HOLLAND, S.F. LOY, AND W.J. VINCENT. A comparison of morning and evening types during maximum exercise. J. Appl. Sports Sci. Res. 6: CABLE, N.T., AND T. REILLY. Influence of circadian rhythms on arm exercise. J. Hum. Mov. Stud. 13: FARIA, J.E., AND B.J. DRUMMOND. Circadian changes in resting heart rate and body temperature, maximal oxygen consumption and perceived exertion. Ergonomics 25: FREIVALDIS, A. Investigation of circadian rhythms on select psychomotor and neurological functions. Ph.D. thesis, University of Michigan, Ann Arbor, GRAEBER, R.C., R. GATTY, F.HALBERG, AND H. LEVINE. Human eating behavior: Preferences, consumption patterns and biorhythms. In: U.S. Army Natick Research and Development Command Technical Report. Natick/TR-78/022. Natick, MA: U.S. Army Natrick Research and Development Command, HILL D.W., D.O. BORDEN, K.M. DARNABY, D.N. HENDRICKS, AND C.M. HILL. Anaerobic work capacity of men and women in the morning and the afternoon. Res. Q. Exerc. Sport. 63:A23 A HILL, D.W., D.O. BORDEN, K.M. DARNABY, D.N. HENDRICKS, AND C.M. HILL. Effect of time of day on aerobic and anaerobic responses to high intensity exercise. Can. J. Sport Sci. 17: HILL, D.W., K.J. CURETON, AND M.A. COLLINS. Circadian specificity in exercise training. Ergonomics 32: HILL, D.W., K.J. CURETON, AND M.A. COLLINS. Effect of time of day on perceived exertion at work rates above and below the ventilatory threshold. Res. Q. Exerc. Sport. 60: HILL, D.W., K.J. CURETON, M.A. COLLINS, AND S.C. GRISHAM. Diurnal variations in responses to exercise of morning and evening types. J. Sports Med. Phys. Fitness. 28: HILL, D.W., K.J. CURETON, M.A. COLLINS, AND S.C. GRISHAM. Effect of the circadian rhythm in body temperature on oxygen uptake. J. Sports Med. Phys. Fitness. 28: HILL, D.W.,AND J.C. SMITH. Circadian rhythm in anaerobic power and capacity. Can. J. Sport Sci. 16: HORNE, J.A., AND O. OSTBERG. A self assessment questionnaire to determine morning-eveningness in human circadian rhythms. Int. J. Chronobiol. 4: HUNTER, J. Of the heat of animals and vegetables. Philos. Trans. R. Soc. Lond. [Biol]. 48: ILMARINEN, J., R. ILMARINEN, O.KORHONEN, AND M. NURMINEN. Circadian variation of physiological functions related to physical work capacity. Scand. J. Work Environ. Health 6: KUHL, J.F.W., J.K. LEE, F. HALBERG, E. HASS, R. GUNTHER, AND E. KNAPP. Circadian and lower frequency rhythms in male grip strength and body weight. In: Biorhythms and Human Reproduction. M. Ferin, F. Halberg, R.M. Richart, and R.L. Vandewicle, eds. New York: John Wiley & Sons, pp LUNDEEN, W.A., G.Y. NICOLAU, D.J. LAKATUA, L. SACKETT-LUN- DEEN, E. PETRESCU, AND E. HAUS. Circadian periodicity of performance in athletic students. In: Chronobiology: Its Role in Clinical Medicine, General Biology and Agriculture. D.K. Hayes, J.E. Pauly, and R.J. Reiter, eds. New York: Wiley-Liss, Inc., pp MELHIM, A.F. Investigation of circadian rhythms in peak power and mean power of female physical education students. Int. J. Sports Med. 14: REILLY, T. Circadian rhythms and exercise: A brief review. In: Ergonomics International. I.D. Brown, R. Goldsmith, K. Coombes, and M.A. Sinclair, eds. London: Taylor & Francis, p REILLY, T.,AND C. BAXTER. Influences of time of day on reactions to cycling at a fixed intensity. Br. J. Sports Med. 17: REILLY, T., AND G.A. BROOKS. Exercise and the circadian variation in body temperature measures. Int. J. Sports Med. 7: REILLY, T., AND A. DOWN. Investigation of circadian rhythms in anaerobic power and capacity of the legs. J. Sports Med. Phys. Fitness 32: REILLY, T.,AND R. GARRETT. Effects of time of day on self paced performances of prolonged exercise. J. Sports Med. Phys. Fitness 35: ROSSI, B., A. ZANI, AND L. MECACCI. Diurnal individual differences and performance levels in some sports activities. Percept. Mot. Skills 57: SHEPARD, R.J. Sleep, biorhythms and human performance. Sports Med. 1: SMITH, C.S., C. REILLY, AND K. MIDKIFF. Evaluation of three circadian rhythm questionnaires with suggestions for an improved measure of morningness. J. Appl. Psychol. 74: TORII, J., S. SHINKAI, S. HINO, Y. KUROKAWA, N. TOMITA, M. HIROSE, S. WATANABE, S. WATANABE, AND T. WATANABE. Effect of time of day on adaptive response to a 4 week aerobic exercise program. J. Sports Med. Phys. Fitness 32: WINGET, C.M., C.W. DEROSHIA, AND D.C. HOLLEY. Circadian rhythms and athletic performance. Med. Sci. Sports Exerc. 17: WYSE, J.P., T.H. MERCER, AND N.P. GLEESON. Time of day dependence of isokinetic leg strength and associated interday variability. Br. J. Sports Med. 28: ZAHORSKA-MARKIEWICZ, B., M. DEBOWSKI, F.M. SPIOCH, J. ZEJ- DA, A. SIKORA, AND A. MARKIEWICZ. Circadian variations in psychophysiological responses to heat exposure and exercise. Eur. J. Appl. Physiol. 59: Acknowledgments I would like to acknowledge Lucinda Cappaert for her invaluable assistance in preparing this manuscript and for her loving support.