Thermoregulatory effect in humans of suppressed endogenous melatonin by pre-sleep bright-light exposure in a cold environment

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 4.5h, and two different ambient temperatures (15 and 27°C) for 2h 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°Ch; thermoneutral: -0.16±0.03°Ch; 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 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.