Sleep, Biorhythms and Human Performance

Abstract

Synopsis:Biological functions show characteristic circadian (≈ 24h), circaseptan (≈7-day), circalunar (≈ 28-day) and circa-annual rhythms. Accurate biological clocks are important to the precise timing of both internal and external events, and also contribute to control processes. Various physiological and psychological factors affecting competitive performance influence the 4 classes of biological rhythm. Normally, there is a synchronisation with external signals (Zeitgebers), but such synchronisation can be upset by rapid time shifts (as in international travel), sleep deprivation, unusual work schedules, use of oral contraceptives, and total environmental control. Sports scientists need to examine both normal times of optimum performance and the rate of adjustment after disturbance ofbiorhythm. Regular, moderate physical activity has surprisingly little influence upon the course of such adaptation. Biological Rhythms: Biological rhythms are important in timing competitions and in arranging travel schedules for athletes. Many biological functions show characteristic circadian (≈ 24h), circaseptan (≈ 7d), circalunar (≈ 28d, in the female) [Minors and Water-house, 1981; Moore-Ede et al., 1982; Webb, 1982], and circa-annual rhythms. Some are truly endogenous, while others are secondary to changes of wakefulness and core temperature, or a response to environmental indicators of time (Zeitgebers). If normal time clues are removed, different individuals develop circadian cycles varying in length from 24 to 26 hours. Normally, cycles become synchronised with the solar rhythm of illumination and social Zeitgebers. The primary oscillators lie in the hypothalamus (supraoptic chiasmata and ventromedian or lateral nuclei). Functional Importance: Biorhythms provide an internal clock that enables the coordination of rapid physiological processes and the precise timing of external events. Both attributes are vital to the skilled competitor. Longer intervals, e.g. the duration of sleep and (in animals) seasonal cycles, are also estimated with remarkable accuracy. High frequency oscillations provide the basis of fine control in many functions, ranging from focus of the eye and maintenance of balance to the adjustment of ventilation to increased CO2 production; however, it is less certain that circadian cycles contribute to normal control processes. Changes of Physiology and of Performance: Arousal is maximal in the afternoon, with associated improvements in pattern recognition, reaction speed and muscle force. Perceived effort falls, fatigue is lessened and all-out effort is better tolerated. Because international competition is itself arousing, laboratory findings may not indicate any real advantage of competitive performance. Body temperature peaks in the late afternoon. Although this is in some respects the equivalent of a ‘warm-up’ it does not influence thermoregulation when exercising in the heat. Heart rate follows arousal and core temperature, so that PWC170 and predicted maximum oxygen intake reach a minimum in the afternoon. Respiratory responses to effort are also less in the afternoon. Metabolic efficiency changes little if allowance is made for diurnal variations of body mass. Any diurnal changes of maximum oxygen intake are small, and even their direction is disputed. Most authors find competitive performance is best in the late afternoon. Physical working capacity as predicted from heart rate (PWC170) shows some circaseptan variation, with the lowest values on Saturdays. In women, the rise of core temperature in the second half of the oestrous cycle has little impact on maximum oxygen intake or anaerobic threshold; indeed, body temperature rises more if exercise is undertaken in the luteal phase. The premenstrual increase of body hydration has a small adverse effect on physical working capacity. Associated sensations may impair both skilled and all-out performance, although muscle force is often increased. Competitive results are uninfluenced by menstruation in the majority of athletes. Primary concerns are hygiene during the early stages of menstrual flow, and the risks of accidents or poor teamwork during premenstrual tension. Humans show few well-established circa-annual rhythms; if the climate is severe, curtailment of activity in the winter can lead to some loss of fitness, while in some cultures seasonal changes of performance can be traced to participation in specific sports programmes. Disturbances of Biorhythm: Diurnal rhythms are disturbed by shifts of time zone (latitudinal air travel) and sleep deprivation. Because circadian cycle length usually exceeds 24 hours, east to west travel is tolerated better than the reverse. There is difficulty in adjusting to a shift >5 hours, and at least 7 days should be allowed for resynchron-isation. Arousal is generally poor until adjustment is complete, and visiting athletes are at a particular disadvantage in the afternoons. Prior desynchronisation and Zeitgeber reinforcement speed adjustment, while short-acting sedatives that do not interfere with REM sleep may help re-establish appropriate cycles of wakefulness. Sleep deprivation is stressful per se, but additional effects may arise from physical fatigue and emotional stress. Resting heart rate and respiratory rate fall, while the resultant acidosis causes some expansion of plasma volume. The heart rate response to submaximum exercise is reduced, but so also are Harvard step test scores, all-out performance, and maximum oxygen intake. Muscle tension tends to decline unless the subject makes strenuous efforts to sustain performance, and there is some decrease of isokinetic strength. Psychomotor performance shows occasional lapses of attention. Accuracy is lost in team events, and self-paced tasks are performed more slowly; psychophysiological tests may also reveal a greater expenditure of effort in order to sustain normal performance. Athletic times usually deteriorate, although it may be difficult to distinguish the effects of sleep deprivation and fatigue. There also seems to be a progressive dampening of normal circadian rhythms. The compression of the working week, use of oral contraceptives and the elimination of seasonal changes in man-made environments undoubtedly disturb circaseptan, circalunar and circa-annual rhythms, but there has been little investigation of functional consequences. The fluid retention resulting from regular oral contraceptive use may help cardiovascular performance, but muscular strength is apparently worsened. Interactions with Exercise: Moderate physical activity should theoretically help synchronisation with new Zeitgebers, particularly if arousal is stimulated at appropriate times during the day In practice, added exercise has little impact upon physiological responses to sleep deprivation, while some authors have actually seen a worsening of psychomotor performance when the effects of sleep deprivation were compounded by vigorous physical activity. Cycles of heavy physical activity play a large part in creating circaseptan rhythms. On the other hand, endurance training may suppress normal menstrual cycles. Conclusion: Physiological and psychological biorhythms have sufficient influence upon performance that they merit close study; indeed, the rapid time shifts associated with translatitudinal travel have already influenced the outcome of some major international competitions. © 1994, ADIS Press Ltd. All rights reserved.