Some of us are morning people, others prefer to wait until the afternoon and evening before heading to the pool or hitting the road. But is there any physiological benefit to training at a particular time of day? Put another way, can Circadian rhythms influence your training and performance?
By Tim Mickleborough
This is a very complex and little-understood area of physiology. The 24-hour rhythm of light and dark is a major influence upon the physiology, biochemistry and ecology of most plants and animals. By convention, most of us regulate our activities so that night is for sleep, the daytime for work and the evenings for leisure. We are, both by design and desire, rhythmic creatures. Rhythms having cyclic periods of about 24 hours are termed circadian rhythms. The recent developments of miniaturized portable data loggers and radioimmunoassay techniques for the detection of hormones and other substances in very small samples of body fluids have enabled us to determine circadian rhythms with comparative ease.
It has been shown that body temperature, blood pressure and urinary potassium loss are greater during the daytime (12 p.m.-6 p.m.) and lower during the night, with minimum values in the small hours. Exceptions to this pattern are peak values of plasma cortisol, testosterone and growth hormone during hours of nocturnal sleep.
Changes in lifestyle, as after a time-zone transition or shift to a night work schedule, don’t result in immediate changes in the rhythms of body temperature or plasma cortisol. For example, volunteers living in isolation chambers (where time cues from the outside, light and noise are excluded) have maintained their normal 24-hour temperature variations. This result indicates the presence of an endogenous component or timing mechanism that is metaphorically called a body clock. Circadian rhythms can affect the physiological determinants of exercise performance. Exercise or performance rhythms conform closely in phase with body temperature and the level of arousal.
Circadian rhythms may be influenced by environmental factors such as temperature, light and dark. These rhythms therefore have implications for elite athletes and for people performing exercise for health purposes who are seeking optimal times of the day for training. They also have implications for sports medicine personnel who may want to consider the impact of the time of day on joint stiffness and pain perception. Most athletic records are set in the late afternoon or evening. This partly reflects the fact that record attempts are usually scheduled for evening meetings when the environmental temperature is more favourable. Nevertheless, athletes tend to prefer evening contests and consistently achieve their top performances at this time of day.
Sports contests are not amenable to the types of manipulation demanded by experimental design. Therefore, researchers tend to concentrate on the effects of time of day on performance in time trials or simulated contests. In the first investigation into diurnal variation in performance, six runners, three weight throwers and three oarsmen performed better in the evening than in the morning (see ref. 1). Swimmers have produced faster times over 100 m at 5 p.m. compared with 7 a.m. in three out of four strokes studied (see ref. 2) and the speed of running in a five-minute test varied closely with the circadian curve in body temperature (see ref. 3).
There is a time window close to the peak in core body temperature in which optimal performance in sports involving gross motor tasks can be attained. This window can extend for four to six hours provided that meals and rests are suitably fitted in during the daily routine. Sports that require fast explosive efforts tend to peak earlier and may be dependent on the sleep-wake clock rather than body temperature. Consequently, practices where skills have to be acquired should be conducted early in the day or around midday, but more severe training drills and “pressure training” practices are best timed for later in the day.
Many physiological parameters are known to show circadian rhythmicity. These include metabolic, cardiovascular and endocrine functions. The metabolic functions showing cyclical changes include oxygen consumption, carbon dioxide production and ventilation.
A peak in heart rate occurs earlier in the day than the peak in core body temperature, but is close to the phase of the rhythm in circulating catecholamines, or hormones that govern wakefulness and physiological arousal. The peak-to-trough heart rate variation at rest is about 8 beats per minute; athletes and coaches should take this into account when resting pulse rate is used as an index of either training state or overtraining. The catecholamine rhythms are probably closely related to changes in arousal.
Physiological rhythms detected at rest may be obliterated or attenuated, or maintained or amplified, under exercise conditions. The most robust rhythm seems to be minute ventilation (liters of oxygen expired per minute), which is amplified at light and moderate exercise intensities. This may partly explain the mild dyspnea, or shortness of breath, sometimes associated with exercising in the early morning.
Studies of the maximal heart rate during exercise have consistently shown an influence of the time of day. A circadian rhythm in recovery heart rate has been shown to occur soon after maximal exercise ceases. Therefore fitness indices, such as the Harvard test score, could contain an error as large as 5 percent because of the time of day the test is performed. This also means that self-monitoring of post-exercise pulse rates is subject to at least this degree of error.
Exercise to voluntary exhaustion at intensity close to VO2max exhibits circadian variation. Subjects performing a high-intensity cycle ergometer test to exhaustion exercised for longer in the evening (10 p.m.) than in the morning (6:30 a.m.), with the mean values being 436 and 260 seconds, respectively. The subjects also tolerated higher blood-lactate levels in the evening as a result of the increase in total work performed at that time (see ref. 4). However, it is possible that rhythms in vigorous exercise may also be due to a combination of motivation and psychological drive that is reflected in anaerobic power output.
1. Conroy, R. T. W. L. and O’Brien, M. Diurnal variation in athletic performance. Journal of Physiology 1974; 236: 51P.
2. Rodahl, A, O’Brien, M and Firth, P. G. R. Diurnal variation in performance of competitive swimmers. Journal of Sports Medicine and Physical Fitness 1976; 16: 72-6.
3. Reilly, T. Human circadian rhythms and exercise. Critical reviews in Biomedical Engineering 1990; 18:165-80.
4. Reilly, T and Baxter, C. Influence of time of day on reactions to cycling at a fixed high intensity. British Journal of Sports Medicine. 1983; 17: 128-30.