Nycthemeral Variations in Core Temperature and Heart Rate: Continuous Cycling Exercise versus Continuous Rest

Abstract

Circadian rhythms have formed the subject of many researches in man during bed rest or usual routine, but have been little studied during continuous and sustained physical exercise. This study deals with the influence of time of day on biological markers in competitive cyclists during continuous physical exercise versus continuous rest. Ultra-distance cyclists were studied over a 24 h period (13:00 to 13:00 h the next day) in the laboratory. The subjects were requested to maintain a constant speed (set at 65% - 70% of their maximal aerobic speed obtained during a preliminary test) on their own bicycles which were equipped with home trainers. Workload, core temperature and heart rate were monitored continuously. The same measures were also recorded while the athletes were resting awake until 13:00 h the next day. Results show that in both situations, core temperature and heart rate exhibited significant circadian variations (p < 0.001). Furthermore, during exercise, an accentuation of amplitude and mean of every rhythm (p < 0.05) with a phase lag (p < 0.05) were observed. Despite a strenuous and continuous physical exercise requiring special physiological adaptations, the rhythmic variations observed at rest persisted, which highlighted the influence of biological clocks.

Effect of a 24-h continuous walking race on cardiac autonomic control

“…This temporal pattern was evident both during and after exercise. Also, Callard et al (2001) found that the circadian rhythm is preserved during a 24-h cycle ergometer session at 65% of maximal oxygen uptake. Although the effects of light/dark cycle and exercise were not dissociated, the present study shows that the nycthemeral rhythm is slightly delayed compared to resting conditions in agreement with studies by Callard et al (2001), Youngstedt et al (2002) and Baher et al (1999).…”

“…HR during exercise was lower at night (3-5 beats min -1 ) irrespective of work rate. Callard et al (2001) found that HR and core temperature exhibited nycthemeral variations despite continuous exercise performed on a cycle ergometer at constant load during 24 h. However, Cohen (1980) argued that HR during maximal exercise does not change with the circadian rhythms.…”

“…As in the study of Callard et al (2001), the present work hypothesizes that during a 24 h continuous walking race HR would remain influenced by the circadian rhythm with a minimum during the night and a maximum level in the early evening. Such a competition represents a severe stress to the cardiovascular system, which could be assessed by spectral analysis of successive RR series.…”

Exercise as a synchroniser of human circadian rhythms: an update and discussion of the methodological problems

“…A higher rhythm amplitude in body temperature for the Wt/active subjects was explained by a lower nocturnal minimum. Callard et al (2001) also found that circadian rhythm amplitudes of core temperature and heart rate were ampliWed during 24 h of continuous self-selected exercise compared to the same period of rest. Mermin and Czeisler (1987) found that, after a bout of hyperthermia-inducing exercise during the day, there is a proportional drop in body temperature below that observed during sleep after no exercise is taken.…”

Morning or evening training: effect on heart rate circadian rhythm

“…Therefore, not only physical exercise as such, but also the time of day in which it is carried out, is an important determinant in regulating the equilibrium of the autonomous nervous system. Callard et al [27] demonstrated that MESOR and amplitude are altered by protracted training like continuous training for over 24 hours (with 20-min breaks): they showed an increase in both the average HR and the amplitude of the rhythm. Our analysis of 24-h HR data, deprived of the training session HR data, did not show any modification in either MESOR or amplitude of the oscillation.…”