Abstract
BackgroundBlue-enriched white light at night has the potential to delay the circadian rhythm in daily life. This study was conducted to determine whether the use of high correlated color temperature (CCT) light at home at night is associated with delay of sleep timing in university students.MethodsThe survey was conducted in 2014–2015 in 447 university students in Japan and 327 students in China. Habitual sleep timing and type of CCT light at home were investigated by using a self-administered questionnaire. The Japanese students were significantly later than the Chinese students in bedtime, wake time, and midpoint of sleep. They were asked whether the lighting in the room where they spend most of their time at night was closer to warm color (low CCT) or daylight color (high CCT). The amount of light exposure level during daily life was measured for at least 1 week by the use of a light sensor in 60 students in each country.ResultsThe percentages of participants who used high CCT lighting at night were 61.6% for Japanese students and 80.8% for Chinese students. Bedtime and sleep onset time on school days and free days were significantly later in the high CCT group than in the low CCT group in Japan. The midpoint of sleep in the high CCT group was significantly later than that in the low CCT group on free days but not on school days. On the other hand, none of the sleep measurements on school days and free days were significantly different between the high CCT and low CCT groups in China. Illuminance level of light exposure during the night was significantly higher in Japanese than in Chinese, but that in the morning was significantly higher in China than in Japan.ConclusionsThe use of high CCT light at night is associated with delay of sleep timing in Japanese university students but not in Chinese university students. The effects of light at night on sleep timing and circadian rhythm may be complicated by other lifestyle factors depending on the country.
Highlights
Blue-enriched white light at night has the potential to delay the circadian rhythm in daily life
Sleep habits were compared in the high correlated color temperature (CCT) group and the low CCT group for both Japanese students and Chinese students after adjusting for sex and age
There was no significant correlation between illuminance level and any of the sleep measurements for both the Japanese students and Chinese students
Summary
Blue-enriched white light at night has the potential to delay the circadian rhythm in daily life. On the other hand, evening light exposure delays the phase of circadian rhythms. In addition to the effect on the circadian rhythm, light at night has an impact on suppression of melatonin secretion [3] and an increase in arousal level [4]. These physiological and endocrinological effects are known as non-visual effects of light and/or non-imageforming effects of light [5,6,7]. Excessive exposure to light at night has negative effects on human sleep and circadian rhythm [8]
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