CORE TEMPERATURE CIRCADIAN RHYTHM DURING 35 DAYS HORIZONTAL BED REST Petra Golja 1,2, Ola Eiken, Samuel Rodman 2,3, Brane Sirok 3 and Igor B. Mekjavic 1,2 Swedish Defence Research Agency, Karolinska Institutet, Stockholm, Sweden; 1 Institute of Biomedical and Biomolecular Sciences, Department of Sports and Exercise Science, University of Portsmouth, UK; 2 Institute Jozef Stefan & 3 Faculty of Mechanical Engineering, University of Ljubljana, Ljubljana, Slovenia BACKGROUND Mean body temperature and the amplitude of its daily oscillations have been reported to decrease during prolonged bed rest (1,2). Such changes in mean body temperature may reflect an imbalance between heat production/gain, and heat loss mechanisms. To evaluate how changes in heat loss might affect core temperature circadian rhythm, the present study monitored the pattern of heat loss from the skin using infra-red thermography during 35 days of bed rest. RESULTS There was no significant difference in either mean daily aural temperature between day 1 ( °C) and the last week of bed rest ( °C), or the magnitude of diurnal oscillations of aural temperature (day 1 of bed rest: 1.0  0.3°C; last week of bed rest: 1.2  0.4°C) Skin temperature, as determined with infrared thermography, decreased significantly (P<0.005) during bed rest. The most pronounced decrements were at the tip of the toes (Figure 1), where the maximal change amounted to -6.1  4.3°C, by the fourth week of bed rest. METHODS Ten healthy males participated in a 35-day horizontal bed rest. Tympanic temperature (Tty) was measured on the first day of the bed rest period, one week later and in the last week of the bed rest. Tty was measured hourly from 8 AM to 11 PM with an infrared aural thermometer. Infrared thermograms were taken at weekly intervals at the same time of the day. Solar radiation and drought were minimised during the measurements. CONCLUSIONS Prolonged (35 day) bed rest had no effect on diurnal core temperature rhythm. The observed decrease in skin temperature during bed rest, reflecting peripheral vasoconstriction, was most pronounced in distal regions. Peripheral vasoconstriction reduces body heat loss and may therefore aid in maintaining a normal deep body temperature during the course of prolonged bed rest, despite the reduction in basal metabolic rate that has been reported to occur during hypokinesia (3). ACKNOWLEDGEMENT The authors are indebted to the personnel at the Valdoltra Orthopaedic Hospital in Ankaran (Slovenia). day 1 week 1 week 4 recovery REFERENCES 1. Lkhagva L. (1980) Circadian rhythm of human body temperature in antiorthostatic position. Kosm. Biol. Aviakosm. Med. 14: Winget C. M., Vernikos-Danellis J. et al. (1972) Circadian rhythm asynchrony in man during hypokinesis. J. Appl. Physiol. 33: Katkovskiy B. S. (1967) Human basal metabolism during prolonged bed rest. Kosm. Biol. Aviakosm. Med. 1: Figure 1. Representative infra-red thermograms (N=1) during 5 weeks of horizontal bed rest and after one month of recovery.