Light-induced Melatonin Suppressions by 1000-Hz Flickering and Nonflickering Blue Light Conditions

Night bright light is known to suppress melatonin secretion. However, novel photoreceptors, named intrinsically photosensitive retinal ganglion cells, are mainly responsible for projecting dark/bright information to the superchiasmatic nucleus and thus regulating the circadian system. It has been shown that a 100-Hz flickering light suppresses melatonin secretion more than the nonflickering one even if the dose of the light conditions were the same. In this study, we evaluated light-induced melatonin suppression under the 1000-Hz flickering and nonflickering light conditions. Twelve male participants between the ages of 20 and 24 years (mean±standard deviation=22.6±1.7 years) were exposed to three light conditions (dim, 1000-Hz flickering, and nonflickering blue light) from 1:00 A.M. to 2:30 A.M., and saliva samples were obtained just before 1:00 A.M. and at 2:30 A.M. A repeated measures t-test with the Bonferroni correction showed that the melatonin concentrations after the light exposure was significantly higher than before the light exposure under dim light conditions, whereas there was no significant difference between the concentrations under 1000-Hz flickering and nonflickering light conditions. These observations suggest that the 1000-Hz flickering light do not have acute impacts on human circadian system as induced by the 100-Hz flickering one.