Light conditions during sleep period and sleep-related lifestyle in Japanese students.

The effects of light conditions during night sleep on sleep habits and morning-evening preference were studied in Japanese students (18-28 years old). Students who usually used fluorescent or non-fluorescent light on the room ceiling, wall or desk preferred to have a daytime nap significantly later (Mean: 14:50 h) than those who used no light (13:34 h). Students who used no curtain or a half-transparent lace-curtain showed shorter sleep latency (duration from going-to-bed to sleep-onset) than those who used a curtain which shuts off lights from outside. Light conditions during the middle of night and early in the morning may affect the timing of sleep in Japanese students based on the circadian system.