Circadian variations in response to cold.

Abstract

Six young adult males were exposed to 10°C ambient temperature for 2 hours by day and by night. Measurements were made of metabolic heat production, mean skin temperature and rectal temperature. At 28°C, prior to cold exposure, skin and rectal temperatures tended towards lower and higher values by day than by night, while the difference in heat production was equivocal between day and night. At 10°C, mean skin temperature and heat production tended to increase by day. A significantly higher rectal temperature was observed in the cold as well as a higher initial rise by day than by night. Changes of heat production, mean skin temperature and rectal temperature at 10°C with respect to those at 28°C were evaluated by three dimensional analysis. There were significant effects both for circadian and individual variations. Individual variation was more consistent than circadian variation according to F ratio. The ratio of the increase in heat production to the decrease in mean skin temperature was also statistically higher by day than by night. These results are attributable to the increased lower critical skin temperature and set point or reference temperature in the central nervous system by day. Circadian variation in response to cold corresponds to adapta tional alteration among individuals and ethnic groups, i. e. metabolic type by day and insulative or habituative type by night.