The aim of this study was to show the resistance and persistence of the circadian rhythm of temperature (T degree) and the sleep quality of athletic subjects and sedentary subjects engaged in night work, and attempt to explain the mechanisms that influence these differences. The effects of night work on biological rhythms have been studied extensively in the past few years. The contradictory situations for the night workers irrefutably affect their biological systems. Individuals with high amplitudes in their circadian rhythms have been found to be more tolerant to shift work and this results in a greater stability of circadian rhythms. This seems beneficial in coping with frequent rhythm disturbances. The physical training program seems to improve several mechanisms of the human biological system: amplitudes of circadian rhythms were increased and the circadian rhythm period was more resistant to an environment extreme (night work, shift work, sleep deprivation, or jet lag). To test this hypothesis, athletes and sedentary subjects who were engaged in regular night work were selected in the PSA Peugeot Citroën Automobiles Group in French Normandy country. The circadian rhythm of the T degree for both groups was studied with a specific methodology and with extensive spectral analysis, especially the spectral elliptic inverse method. Study models of the rhythm of the T degree were determined and the characteristic parameters were exposed. A complementary actigraphic study showed the physical training program's effects on the sleep quality. The results revealed a large stability in the rhythm of circadian variation of T degree for the athletes: the amplitude was still large but for the sedentary subjects the amplitude of the T degree decreased and it was difficult to adjust a period on the rhythm of T degree. The stability and persistent quality of the athletes' circadian rhythm was confirmed. We observed that the actigraphic sleep was greater for athletes than for sedentary subjects, and the acrophase time for the athletes was later than for the sedentary subjects during the night shift.
