The effect of daytime light conditions on the sleep habits and morning-evening preference of Japanese junior high school students (415 girls and 411 boys; age range, 12-15 years old) was studied. Students who were outdoors during the short break between classes or their lunch-time break were more morning-type people than those who remained indoors. Students who shut out the light from outside showed longer subjective sleep latency and appealed more shallow sleep rather than those who did not. Sunlight can be an important factor for the timing of sleep based on the circadian system of Japanese young students.