The effect of the usage of a blacked-out curtain on human sleep-wake cycles was studied on four Japanese University students (all men) aged 20–23 years. Activities and the light intensity of exposure were recorded with an actigraph system continuously for 6 weeks. The curtains in their rooms that they were using were replaced with blacked-out ones after the first 3 weeks. The delay or irregularity of sleep phase occurred in three of four students. The shutting out of morning light by the blacked-out curtain seems to have significant effects on sleep-wake habits in humans.