Abstract
BackgroundBright nocturnal light has been known to suppress melatonin secretion. However, bright light exposure during the day-time might reduce light-induced melatonin suppression (LIMS) at night. The effective proportion of day-time light to night-time light is unclear; however, only a few studies on accurately controlling both day- and night-time conditions have been conducted. This study aims to evaluate the effect of different day-time light intensities on LIMS.MethodsTwelve male subjects between the ages of 19 and 23 years (mean ± S.D., 20.8 ± 1.1) gave informed consent to participate in this study. They were exposed to various light conditions (<10, 100, 300, 900 and 2700 lx) between the hours of 09:00 and 12:00 (day-time light conditions). They were then exposed to bright light (300 lx) again between 01:00 and 02:30 (night-time light exposure). They provided saliva samples before (00:55) and after night-time light exposure (02:30).ResultsA one-tailed paired t test yielded significant decrements of melatonin concentration after night-time light exposure under day-time dim, 100- and 300-lx light conditions. No significant differences exist in melatonin concentration between pre- and post-night-time light exposure under day-time 900- and 2700-lx light conditions.ConclusionsPresent findings suggest the amount of light exposure needed to prevent LIMS caused by ordinary nocturnal light in individuals who have a general life rhythm (sleep/wake schedule). These findings may be useful in implementing artificial light environments for humans in, for example, hospitals and underground shopping malls.
Highlights
Light is thought to have the most powerful impact on the human circadian rhythm and sleep/wake cycle
Post hoc comparison revealed significant melatonin decrements after nighttime light exposures under day-time dim, 100 and 300 lx light conditions
There were no significant differences in melatonin concentrations between night-time pre- and post-light exposure under day-time 900 and 2700 lx light conditions; melatonin concentrations significantly decreased after night-time light exposure under day-time dim light conditions
Summary
Light is thought to have the most powerful impact on the human circadian rhythm and sleep/wake cycle. Humans adjusted their circadian rhythm to match the natural 24 h bright/dark cycle (i.e. the earth’s rotation). The modern light environments possibly cause the Bright nocturnal light has been known to suppress melatonin secretion [1,2,3,4], and the International Agency for Research on Cancer (IARC) has classified work shifts that involve circadian disruption as a probable carcinogenic factor (Group 2A). Bright nocturnal light has been known to suppress melatonin secretion. Bright light exposure during the day-time might reduce light-induced melatonin suppression (LIMS) at night. This study aims to evaluate the effect of different day-time light intensities on LIMS
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