Thermoregulatory Effect in Humans of Suppressed Endogenous Melatonin by Pre-Sleep Bright-Light Exposure in a Cold Environment

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 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.

Crosstalk Between Environmental Light and Internal Time in Humans

“…This inverse relationship could be explained by the alerting properties of light through sympathetic activation inducing blood-vessel constriction and, in turn, reduction in skin temperature (Buijs et al, 2003). Other authors have demonstrated the short-term effect of bright-light exposure on reducing melatonin and sleepiness and on increasing alertness, heart rate, and core temperature (Cajochen et al, 2005;Ishibashi et al, 2010;Phipps-Nelson et al, 2009). Laboratory studies have suggested that short, intermittent periods of exposure to bright light, such as those characteristic of our modern lifestyle, may have a much greater impact on circadian entrainment than has previously been recognized (Hebert et al, 1998;Okudaira et al, 1983;Savides et al, 1986).…”

Combined effects of environmental factors on human perception and objective performance: A review of experimental laboratory works

“…Forty‐five papers have been finally collected to review the most recent studies on combined effects of environmental factors upon human performance and comfort. Details are provided in the Appendix S1 (SI.2) and in particular in Table SI.1.…”

Interactions between the perception of light and temperature

“…Fewer studies have been carried out on the direct effect of light intensity on thermal sensation and thermal comfort. Two studies found a cooler thermal sensation under low light intensity as compared to bright light, while the third experiment reported no significant effect of light intensity on thermal comfort and thermal sensation . Additionally, changes in thermal sensation caused by the intensity of light may be related to changes in core body temperature .…”