Circadian Periodicity of Plasma Cortisol Levels in Members of Japanese Antarctic Research Expedition in Antarctic Region

Abstract

With collaboration of the members of the 14th Japanese Antarctic Research Expedition (JARE), circadian rhythm of plasma cortisol levels was studied in 4 different seasons during a year. Blood specimens were obtained from 30 members at 22:00, 08:00, 12:00, 18:00 on each experimental day. Time-study was performed on the same days for all members. They had arbitrary daily schedules and sleeping times, except for several members in charge of machine maintenance, 3 weathermen and 2 correspondents, who followed their own fixed schedules. "Normal" circadian rhythm was observed on the days in March and September which have the same day-night cycles as in Japan. The rhythm was disturbed in June, the month of darkness throughout the day. In December, the season of midnight sun, the nyctohemeral rhythm was maintained although plasma cortisol levels were slightly elevated. The members of the machine maintenance group had "normal" rhythm at every period. The nightworkers had inverse rhythm at 4 different period. The circadian rhythm reexamined 4 days after changing the sleep-wake cycle showed the time shift according to the new sleep-wake period. These results suggest that the circadian rhythm of plasma cortisol levels depends on the sleep-wake cycle rather than on the day-night cycle.