| Feature | No.43 January 2005 |
Chronobiology and Sports Performance Thomas Reilly, United Kingdom |
Abstract Chronobiology is the scientific study of biological rhythms which embraces ultradian, circaseptal, circamensal and seasonal rhythms as well as circadian aspects. Circadian rhythms refer to recurring biological cycles over each solar day. Body temperature is regarded as a fundamental variable, having a very strong endogenous component. There is evidence that many human performance measures vary throughout the day in close phase with the temperature curve. Exceptions apply to endurance performances and exercise in high ambient temperatures. The time of day in which sports events are held therefore may influence the quality of performance. There are considerations also for safety and effectiveness of training. Circadian influences are most obvious when underlying rhythms are disturbed, such as in nocturnal shiftwork, sleep deprivation or desynchronisation of rhythms following long-haul flights. Contemporary elite athletes travel regularly across multiple time zones, for purposes of training or competition. Such itineraries induce travel fatigue, including jet-lag when multiple time zones are crossed. Jet-lag refers to a syndrome or cluster of symptoms that reflect desynchronisation of the normal circadian rhythm. Until rhythms are re-synchronised to harmonise with the new local time, performance may be compromised. Jet-lag is more severe and lasts longer the more time-zones that are crossed. Circadian resynchronisation is faster after travelling westwards compared to travelling eastwards. In order to hasten the adjustment to the new local time, the traveller may choose between pharmacological and behavioural strategies. The former includes benzodiazepines, central nervous system stimulants, melatonin, caffeine and other putative chronobiotics. The latter includes alterations of the sleep-wake cycle prior to travelling, but a behavioural strategy must be geared towards the direction of flight, the number of time zones crossed and the time of arrival in the new time zone. Experience of previous long-haul flights seems to provide opportunity for developing appropriate means of coping with jet-lag. 1. Introduction This overview is about chronobiology and sports performance. I hope to explain what the science of chronobiology entails, consider the evidence for rhythms in sport performance and give some examples of how these are manifest. The review must start with some definitions: Chronobiology represents the study of biological rhythms. These may be of different lengths – circadian, circamensal, circannual and so on. Circadian rhythms refer to recurring cyclical changes with the solar day. Biorhythms refer to three periods of 22, 28 and 33 days length respectively but this theory has no scientific basis. A cycle is styled by cosinor analysis. A circadian cycle has a period 24 h in length, an acrophase at time of peak, and a trough 12 hours later (see Figure 1, top). A biological marker is the rhythm in rectal temperature which shows a peak close to 18:00 hours before it starts to decline as night-time approaches (Figure1, bottom). Figure 1. Nomenclature used in rhythm characteristics (top) and the circadian rhythm in rectal temperature (bottom). The main mechanisms responsible for the “body clock” are the suprachiasmatic cells of the hypothalamus – this is the cerebral area also responsible for regulation of body temperature and it has receptors for melatonin. This hormone is secreted by the pineal gland which is produced in darkness and inhibited by light. The optic nerve links perception of light to these cerebral structures. The visual receptors that synchronise the body clock are independent of the classical cones and rods. Consequently, the existence of circadian rhythms is attributed firstly to endogenous rhythms, the so-called body clock. These are linked to the sleep wake cycle, influenced by the pineal hormone and its related neurotransmitter serotonin, body temperature and local timekeepers identified in muscle and other peripheral tissues. External factors that lock the rhythm into a 24-hour period are the alternation of sleep and activity, feeding, external light and darkness, exposure to UV light and exposure to social factors. These are the so-called zeitgeibers or time-givers (see Table 1). Table 1. External signals adjusting circadian rhythm. 1.Pattern of sleep and activity 2.Timing and type of meals 3.Alternation of daylight and darkness 4.Exposure to UV light (e.g. direct sunlight) 5.Exposure to social influences The two sources of observed rhythms can be separated by living in a constant routine of unchanging temperature and no physical activity. The result is shown in the changed shape of the curve of rectal temperature and urine volume (Figure 2). It is clear that there remains a classical cosine function in both variables, when the effects of activity are removed. The other approach is to use “forced desynchronisation” which we can do in an isolation chamber within our premises at Liverpool John Moores University. When we do so, it is apparent that rising times and retiring times get progressively later, the body clock regulating its day around a 26-h period and not an exact 24 hours (Figure 3). Figure 2. The circadian rhythms in body temperature (top) and urine flow (bottom). For each variable the difference between the two curves represents the exogenous component of the rhythm. Figure 3. A constant routine facilitates an extension of the circadian rhythms to > 24 h. 2. An endogenous rhythm in performance Evidence for a circadian rhythm in sport performance was provided by Atkinson et al. who showed that cycling performance over a 16.1 km time trial in the morning was improved by light exercise at that time the morning before compared to change in performance at mid-day. This design avoided the criticism of looking at personal best performances owing to a bias in scheduling record attempts. With the difficulty of manipulating competitive events for experimental purposes, researchers have resorted to investigating components of performance. Rhythms are evident over a range of measures including grip strength, leg strength, flexibility and self-chosen exercise intensity (Reilly et al., 1997). The latter is relevant to training since it indicates athletes will naturally pace themselves at a higher exercise intensity later in the day. Rhythms in metabolism persist under exercise conditions, the rhythm in ventilation being stronger than that of O2 . The rhythm in the ventilation equivalent of oxygen at light and moderate exercise intensities is shown in Figure 4. The rhythm in O2 does not show up at O2 max , providing that O2 max is reached. Subjects not displaying evidence of a maximal value were recalled for testing, which is a good incentive to continue exercise to voluntary exhaustion. The data show an obvious error when submaximal heart rate is used to predict O2 max (see Figure 5) or if it is employed for training prescription at different times of day. Figure 4. The ventilation equivalent of oxygen shows a typical circadian rhythm at two submaximal exercise intensities. Figure 5. Estimating O2 max from submaximal heart rate is prone to error. The broken line indicates O2 max actually measured. The strongest evidence for an endogenous rhythm in sports performance comes from two studies of swimmers. These performers habitually train in the morning. Yet power output on an isokinetic swim bench (Reilly and Marshall,1991), and performance in both 100 m and 400 m swims (Baxter and Reilly, 1983) were better in the evening than in the morning. Further evidence is that the reduced performance in the morning is not due to a warm-up effect. Irrespective of whether cyclists warmed up or not, their times were better in the evening than in the morning. The last piece of evidence comes from conditions that are more or less constant but under sleep deprivation. Soccer players performed for 92 hours (Figure 6). There were recurring cyclical changes in activity and in the heart rate responses (Reilly and Walsh, 1981). Figure 6. Cyclical changes in activity over four days of 5-a-side soccer. 3. Some qualifications There are a few caveats to guard against a simplification of the relationship between time of day and performance. First, the menstrual cycle and time of day may interact. This interaction influences both core temperature (which is elevated in the luteal phase) and performance which has a reduced amplitude in the follicular phase (Bambaeichi et al., 2004). Secondly, the temperature response to exercise may vary with time of day. Hence there is a greater rise in temperature in the morning than in the evening. This is manifest in self-chosen exercise intensity, where the pace is gradually increased with exercise duration in the morning (Reilly and Garrett, 1998). Therefore, starting marathon races in the morning may actually be an advantage. A final association here is the combination of components in complex skills such as tennis and factors dependent on both the body temperature curve and on the central nervous system (Figure 7). Whilst power may be best later in the day, accuracy is best in the morning (Atkinson and Spiers, 1998). Figure 7. Serve accuracy and serve velocity in tennis at three different times of day. 4. Desynchronisation So what happens when rhythms are desynchronised? This disruption occurs on travelling across multiple time zones, an opportunity for a natural experiment. The effect known as jet-lag contrasts with travel fatigue, the result of travelling north or south directly. There can be some confusion about the issues of travelling athletes, even amongst international bodies. In a recent position statement of FIMS, there is doubt expressed about the existence of circadian rhythms in exercise performance. Yet recommendations are made on the purported basis of circadian principles. Similarly, some international athletics coaches refused to accept the existence of jet lag five years ago before the Sydney Olympics. This year, to explain the 400 m performance in the UK Olympic trials, jet-lag was deemed publicly to be the cause of a favoured athlete finishing 5th only. So what is the root challenge to the body clock posed by travelling across time zones? We can compare a trip shown eastward and westward from London, the base of Greenwich Mean Time. When travelling east the urge to sleep is displaced to later in the day, so the body clock needs to advance to match local time (Figure 8). In contrast, flying westwards requires a phase delay. Figure 8. The urge to sleep is displaced after travelling across time-zones. Prior to the 1996 Olympics, the U.K. used Tallahassee, Florida as its base. Crossing entailed a 5-hour time-zone transition. In studies of the gymnasts (Reilly et al., 2001), it was clear that the performance curve was shifted to the left and returned gradually towards its reference levels (Figure 9). Figure 9. Restoration of muscle strength over seven days following a flight westwards. This observation was followed up by a study of the effects of a chronobiotic drug that might act directly on the body clock. The argument was that as sleep is an important factor in readjustment, any substance that would help sleep should accelerate re-alignment. This led to the selection of temazepam for experimentation. Those subjects on temazepam derived no added benefit from the drug. At no stage in the re-adjustment was the temazepam response better than placebo. The conclusions were that jet lag symptoms recovered, differed with time of day, were unaffected by temazepam and differed between individuals (Reilly et al., 2001). The next Olympic Games proved more difficult – the question was whether to delay the body clock by 14 h or advance it by 10 h. Either was possible. A stop-over at Singapore would ease jet lag symptoms and guarantee a phase advance. It was deemed logistically too difficult and too disruptive for athletes. A study of support staff indicated that if they arrived in the morning and stayed up, they adjusted by a phase delay. This made the mental adjustment less difficult for them but accentuated their jet-lag. An alternative was to consider another chronobiotic, melatonin. It has opposite effects to light (or perhaps exercise), consolidating the so-called dark pulse. Administered according to guidelines, melatonin proved to be ineffective (Edwards et al., 2000). That led to a resort to behavioural means. The behavioural alternative is to use natural signals provided by light and by exercise. Consider how they might work – light before the trough in body temperature should delay the body clock, after this trough would advance it. So when using the Olympic preparation strategy the plan was to get straight from the airport to bed – sleep in for two mornings. Linked to this approach was the avoidance of training in the morning and avoid napping in the afternoon. This behaviour would anchor sleep in the time zone just departed. This strategy of using behaviour rather than pharmacology seemed to have worked well. 5. Conclusion In summing up – the main messages are: There is indirect evidence of a circadian rhythm in exercise performance. The interactions between time of day and exercise responses are complex. Adverse effects of desynchronisation can be reduced by basing behaviour on chronobiological principles. References Atkinson, G. and Spiers, L. (1998). Diurnal variations in tennis serve. Percept. Motor Skills, 86, 1335-1338. Atkinson, G., Todd, C., Reilly, T. and Waterhouse, J. (2004). Diurnal variation in cycling performance: influence of warm-up. J. Sports Sci., 22 (in press). Bambaeichi, E., Reilly, T., Cable, N.T. and Giacomoni, M. (2004). The isolated and combined effects of menstrual cycle phase and time of day on muscle strength of eumenhorreic females. Chronobiol. Int., 21, 645-660. Baxter, C. and Reilly, T. (1983). Influence of time of day on all-out swimming. Brit. J. Sports Med., 17, 122-127. Edwards, B.J., Atkinson, G., Waterhouse, J., Reilly, T., Godfrey, R. and Budgett, R. (2000). Use of melatonin in recovery from jet-lag following an eastward flight across 10 time-zones. Ergonomics, 43, 1501-1513. Reilly, T. and Garrett, R. (1998). Investigation of diurnal variation in sustained exercise performance. Ergonomics, 41, 1085-1094. Reilly, T. and Marshall, S. (1991). Circadian rhythms in power output on a swim bench. J. Swim. Res., 7(2), 11-13. Reilly, T. and Walsh, T.J. (1981). Physiological, psychological and performance measures during an endurance record for 5-a-side soccer play. Brit. J. Sports Med., 5, 122-128. Reilly, T., Atkinson, G. and Budgett, R. (2001). Effect of low dose temazepam on physiological variables and performance tests following a westerly flight across four time zones. Int. J. Sports Med., 2, 166-174. Reilly, T., Atkinson, G. and Waterhouse, J. (1997). Biological Rhythms and Exercise. Oxford: Oxford Univ. Press. Thomas Reilly Research Institute for Sport and Exercise Sciences Liverpool John Moores University Henry Cotton Campus 15-21 Webster Street Liverpool, L3 2ET United Kingdom http://www.icsspe.org/portal/bulletin-january2005.htm Chronobiology and Sports Performance Thomas Reilly, United Kingdom