Altered sleep–wake cycles and physical performance in athletes

Abstract

Sleep–waking cycles are fundamental in human circadian rhythms and their disruption can have consequences for behaviour and performance. Such disturbances occur due to domestic or occupational schedules that do not permit normal sleep quotas, rapid travel across multiple meridians and extreme athletic and recreational endeavours where sleep is restricted or totally deprived. There are methodological issues in quantifying the physiological and performance consequences of alterations in the sleep–wake cycle if the effects on circadian rhythms are to be separated from the fatigue process. Individual requirements for sleep show large variations but chronic reduction in sleep can lead to immuno-suppression. There are still unanswered questions about the sleep needs of athletes, the role of ‘power naps’ and the potential for exercise in improving the quality of sleep.

Introduction

The sleep–wakefulness cycle is the most discernable of human circadian functions, activity being associated with the hours of daylight and sleep with the hours of darkness. This recurrence on a daily basis is linked with responses of the pineal gland to the environment, its secretion of melatonin being promoted at dusk and inhibited on exposure to morning light. There is a myriad of other biological functions that are knit into a common system of circadian rhythms, cycles in behaviour and in biological functions that recur with a period of about 24 h (circa diem).

Many human performance measures follow broadly the typical circadian curve in body temperature, including for example muscle strength, anaerobic power output, joint-flexibility and self-chosen work-rate [1]. Observations from time-trials in swimming [2] and cycling [3] provide indirect support for an endogenous component to these rhythms in exercise performance. There are suggestions that complex skills tend to peak earlier in the day than do gross motor skills, due possibly to an earlier acrophase in the circadian rhythm in alertness compared to that of body temperature [4], [5]. This separation of central nervous system arousal from alignment with the body temperature rhythm has been attributed, at least in part, to the circadian rhythm in circulatory catecholamines [6] and to the homeostatic drive for recuperation from fatigue due to time since waking from the previous sleep [7], [8]. Indeed, the amount of sleep individuals have acquired in the previous 24 and 48 h has been incorporated into a predictive model for determining fatigue-risk thresholds in normal hours of occupational service [9].

The harmonious co-existence of distinct circadian rhythms cannot be assumed when the normal sleep–wakefulness cycle is disrupted. Such perturbations occur as a result of changes in domestic circumstances that interrupt normal sleep, when anxiety prohibits restful sleep and when operating on nocturnal shift-work. They also apply to travellers on long-haul flights over multiple meridians and to a lesser extent to Muslims fasting during the holy month of Ramadan when eating and drinking are eschewed from sunrise to sunset (see the paper by Reilly and Waterhouse in this special issue). The consequences are usually apparent in mood, alertness and performance [10]. The effects of these disruptions may be more pronounced in athletic activities, particularly ‘adventure’ events where the amount of time allocated for sleep is minimised.

The importance of good quality sleep for sports participants is recognised by practitioners (e.g. [11]). Insights can be gained into the role of sleep by looking at the consequences of disruptions to the sleep–waking cycle and how individuals cope in such conditions. In this review the effects of total sleep deprivation, chronic sleep loss and partial sleep allowances are considered, and results reviewed from both laboratory and field studies. The circumstances inducing jet-lag and nocturnal shift-work are then reviewed along with remedies for counteracting any performance impairments. Sleep disruption in the context of individual differences is discussed prior to setting out guidelines for coping with necessary breaks to normal sleep.