||Bright light at night has been known to suppress melatonin secretion. Photoreceptors, known as intrinsically photosensitive retinal ganglion cells (ipRGCs), project dark/bright information into the superchiasmatic nucleus, which regulates the circadian system. Electroretinograms of ipRGCs show fluctuation that is synchronized with light ON-OFF stimulation. This finding suggests that the flickering condition of light may have an impact on our circadian system. In this study, we evaluate light-induced melatonin suppression under flickering and non-flickering light conditions. Fifteen male subjects between the ages of 20 and 23 years (mean +/- SD, 21.9 +/- 1.9) were exposed to three light conditions (dim, 100-Hz flickering and non-flickering light) from 1:00 a.m. to 2:30 a.m. Saliva samples were taken just before 1:00 and at 1:15, 1:30, 2:00, and 2:30 a.m. Repeated-measure t-test with Bonferroni correction showed a significant decrease in melatonin levels under both 100-Hz and non-flickering light conditions compared to dim light conditions after 2:00 a.m. Moreover, at 2:30 a.m., the rate of change in melatonin level under 100 Hz of flickering light was significantly lower than that under non-flickering light. Our present findings suggest that 100-Hz flickering light may suppress melatonin secretion more than non-flickering light.