Physiological Significance of Cyclic Changes in Room Temperature around Dusk and Dawn for Circadian Rhythms of Core and Skin Temperature, Urinary 6-hydroxymelatonin Sulfate, and Waking Sensation just after Rising

The present study investigated whether room temperature (T_a) cycles around dusk and dawn could influence the circadian rhythm of rectal temperature (T_<core>), urinary 6-hydroxymelatonin sulfate during nocturnal sleep, and subjective assessments of sleep in humans. Six female and two male students served as participants. Two different T_a conditions, cyclic and constant, were established. Two kinds of room temperature were provided to subjects: cyclic T_a (gradual decrease from 27℃ to 24℃ between 1800 and 2200 h and gradual increase from 24℃ to 27℃ between 0300 and 0700 h) and constant T_a (27℃ over 24h). At cyclic T_a, the circadian nadir of T_<core> rhythm was significantly advanced, while T_<core> was significantly lower from 2300 to 0200 h and significantly higher from 0600 to 1000 h. The nocturnal concentration of 6-hydroxymelatonin sulfate in the urine during sleep was significantly higher during cyclic T_a. Waking sensation just after rising was significantly better with cyclic T_a. (Skin temperatures in the extremities - T_a) were significantly higher with cyclic T_a especially during the evening and night. Our results suggest that gradual change of room temperature in the evening and early morning is important in terms of sleep promotion and fresh awakening. It seems probable that mankind has been evolved to have deeper sleep under the influence of cyclic T_a around dusk and dawn.