Nycthemeral variations in core temperature and heart rate: Continuous cycling exercise versus continuous rest

Circadian rhythms have formed the subject of many researches in man during bed rest or usual routine, but have been little studied during continuous a nd sustained physical exercise. This study deals with the influence of time of day on biological markers in competitive cyclists during continuous phy sical 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 t heir maximal aerobic speed obtained during a preliminary test) on their own bicycles which were equipped with home trainers. Workload, core temperatur e and heart rate were monitored continuously. The same measures were also r ecorded while the athletes were resting awake until 13:00 h the next day. R esults show that in both situations, core temperature and heart rate exhibi ted significant circadian variations (p < 0.001). Furthermore, during exerc ise, 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 ph ysical exercise requiring special physiological adaptations, the rhythmic v ariations observed at rest persisted, which highlighted the influence of bi ological clocks.