Diurnal variation of thermal resistance in rats

Effects of photoperiod on heat resistance were studied in 88 rats by observing their survival times in a hot environment (42.5 degrees C). Prior to the experiments individual rats were exposed to a given heat (42.5 degrees C) at a definite time of day and a "predicted survival time" in a given heat in individual rats was obtained. Rats were then divided into eight groups (with nine rats in each group) so as to ensure intergroup homogeneity regarding their predicted survival time and were exposed to heat at different times of day (every 3 h) until they were exhausted. It was found that the heat resistance varied with the time of day. In the eight groups kept under a normal light-dark cycle (L, 0700-1900;D, 1900-0700), heat resistances were observed to be significantly higher in the light phase than in the dark phase. Lethal body temperature was not correlated with the heat resistance. In two other groups (n = 8) kept under conditions reversed from the normal lighting cycle, resistance was higher in the nighttime (corresponding to the light phase when the rats were kept in the reversed lighting cycle) than in the morning (corresponding to the dark phase), these changes being accompanied by a phase shift of the diurnal changes in body temperature.