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Abstract
This study investigated the physiological function of suppressed melatonin through thermoregulation in a cold environment. Interactions between thermoregulation directly affected by exposure to a cold environment and indirectly affected by endogenous melatonin suppression by bright-light exposure were examined. Ten male subjects were exposed to two different illumination intensities (30 and 5000 lux) for 4.5 h, and two different ambient temperatures (15 and 27°C) for 2 h before sleep under dark and thermoneutral conditions. Salivary melatonin level was suppressed by bright light (p < 0.001), although the ambient temperature condition had no significant effect on melatonin. During sleep, significant effects of pre-sleep exposure to a cold ambient temperature (p < 0.001) and bright light (p < 0.01) on rectal temperature (Tre) were observed. Pre-sleep, bright-light exposure led to an attenuated fall in Tre during sleep. Moreover, Tre dropped more precipitously after cold exposure than thermoneutral conditions (cold: −0.54 ± 0.07°C/h; thermoneutral: −0.16 ± 0.03°C/h; p < 0.001). Pre-sleep, bright-light exposure delayed the nadir time of Tre under thermoneutral conditions (p < 0.05), while cold exposure masked the circadian rhythm with a precipitous decrease in Tre. A significant correlation between the Tre nadir and melatonin level (r = −0.774, p < 0.05) indicated that inter-individual differences with higher melatonin levels lead to a reduction in Tre after cold exposure. These results suggest that suppressed endogenous melatonin inhibits the downregulation of the body temperature set-point during sleep. (Author correspondence: [email protected])