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 degrees 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 (T(re)) were observed. Pre-sleep, bright-light exposure led to an attenuated fall in T(re) during sleep. Moreover, T(re) dropped more precipitously after cold exposure than thermoneutral conditions (cold: -0.54 +/- 0.07 degrees C/h; thermoneutral: -0.16 +/- 0.03 degrees C/h; p < 0.001). Pre-sleep, bright-light exposure delayed the nadir time of T(re) under thermoneutral conditions (p < 0.05), while cold exposure masked the circadian rhythm with a precipitous decrease in T(re). A significant correlation between the T(re) nadir and melatonin level (r = -0.774, p < 0.05) indicated that inter-individual differences with higher melatonin levels lead to a reduction in T(re) after cold exposure. These results suggest that suppressed endogenous melatonin inhibits the downregulation of the body temperature set-point during sleep.

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