Field Studies on Circadian Rhythms of Core Temperature in Tropical Inhabitants Compared with Those in European Inhabitants

Abstract

In order to know the characteristics of circadian rhythms in core temperature in tropical inhabitants, we measured rectal temperatures every 10 min for 50 h in 6 Vietnamese, 20–22 years old (5 males and 1 female) under natural conditions. Ambient temperatures ranged from 33 to 36°C. These data obtained in tropical inhabitants were compared with those in Polish inhabitants. The participants in both groups were usually sitting during wakefulness and lying in bed during sleep. The results obtained are summarized as follows: (1) The average maximum value was 37.7°C, which was significantly higher than in the Europeans; (2) The average minimum value was 36.2°C, which was lower; (3) A range of oscillation was 1.5°C, which was clearly greater than in the Europeans. The higher maximum value of core temperature, which was actively regulated under warm temperature, seemed of adaptive significance in order to reduce water consumption. A greater rage of oscillation in tropical Vietnamese people might have ecological significance for efficient acclimatization in the warm environment, suggesting that the setpoint of core temperature could show a greater range of oscillation.