Circadian cycles of central temperature in hot climate in man

In order to appreciate eventual changes of the circadian rhythm of the body core temperature induced by hot climates, we measured for 24 h periods in summer (June) and in winter (December), in Senegal (latitude, 25 degrees north), the central temperature in two groups of 45 and 58 young soldiers living permanently in the Sahelian area. The subjects were free-running but without any strong activity during the experiments. Rectal temperatures were measured at rest each 3 h, in natural environment: in winter ambient temperature varied from 21 to 25 degrees C and in summer from 28 to 42 degrees C. We observed no change in the shape of the circadian cycle: minimal values were seen at 3 AM and maximal at 3 or 6 PM. But during the hot season, we observed increase of the mesor (36.85 and 37.10 degrees C, in december and june respectively) and of the amplitude of the nycthemeral variation (0.85 and 1.37 degrees C respectively). These results demonstrate a clear adaptation to high ambient temperatures in man, similar to that observed in other mammals.