Review
Circadian aspects of body temperature regulation in exercise

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Abstract

Internal and external factors contribute to resting core temperature and affect thermoregulation. Also, a robust circadian rhythm exists, implying that the body is in “heat-gain” or “heat-loss” modes at different times during the 24 h. Moreover, many variables associated with exercise, and the body's capacity for exercise, show circadian variation. All these factors contribute to circadian changes in thermoregulation during exercise. Attention is focused on responses at the onset of exercise, “critical temperature”, and recovery after exercise. Practical implications of circadian changes in thermoregulation during exercise include ergogenic aids and inter-individual differences, including those due to gender, age and acclimatisation.

Section snippets

Background

In this review, our main aims are to demonstrate the impact of circadian rhythms on thermoregulation and establish some consequences for physiological responses to exercise. A further aim is to place the role of ergogenic practices that manipulate body temperature in context of the circadian milieu. Finally, we consider how such fluctuations differ between males and females, individuals’ fitness and acclimatisation status, and exercise dimensions.

Circadian rhythms—general principles

The daily variations observed in circadian rhythms include endogenous (due to the “body clock”) and exogenous components. The exogenous influences consist of alterations in activity in accordance with the sleep-wake cycle, and changes in ambient temperature, as described above. The relative size of the endogenous and exogenous components of a circadian rhythm can be estimated by the “constant routine” protocol and “purification” methods. In the constant routine protocol, subjects remain awake

Exercise performance

Maximal exercise of short duration demonstrates circadian rhythmicity closely in phase and shape to that of core temperature. This similarity applies to isometric force (Coldwells et al., 1991), anaerobic power and anaerobic capacity (Reilly and Down, 1986, Reilly and Down, 1992), peak isokinetic torque, grip strength, joint flexibility and many other measures (Reilly, 1990). The rhythms are observable in simulations of competition and time-trials in cycle ergometry (Reilly and Baxter, 1983),

Interactions between the response to exercise and thermoregulation

Because similar thermoregulatory mechanisms are involved in the control of the circadian rhythm of resting core temperature and in reducing the temperature rise during exercise, interactions between them might exist (Fig. 2), particularly as the body clock and the thermoregulatory centres are in anatomical proximity within the hypothalamus (Reilly et al., 1997). Evidence supporting these predictions will now be considered.

Individual differences

There are individual differences in thermoregulatory responses to exercise. Among the factors involved are effects of gender, age and behavioural type. Responses also vary with training, acclimatisation and seasonal variations. These effects may have practical consequences for exercise and for circadian characteristics.

Overview

Models of thermoregulation must take into account the influences of circadian rhythms, independent of environmental factors. It seems that the set-point at rest is altered in a circadian fashion, reflecting the cyclic changes between the need for rest and for activity. As a result, the body temperature has been used as a marker of endogenous circadian rhythms and to track adaptations to disruptions such as occur in nocturnal shift work or travelling across multiple time zones. The circadian

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