A sleep and life-style survey of Japanese high school boys: Factors associated with frequent exposure to bright nocturnal light

Shortened nocturnal sleep is a growing social phenomenon in industrialized countries. The Japanese high school student, with an average of 6.3 h sleep, is an extreme example. A recent feature in Japanese adolescents' life-style is a pattern of frequent exposure to bright nocturnal light, which is believed to bear considerable impact on their sleep habits. The details of this impact, however, remain unspecified and invite clarification. To determine the factors associated with this particular behavioral pattern we conducted a cross-sectional epidemiological survey of high school boys using a questionnaire on their sleep habits, sleep problems, subjective daytime complaints and life-styles, including the frequency of their nocturnal visits to convenience stores. A total of 2 160 valid answers were analyzed (response rate 97.1%). We divided the students into four groups according to the frequency of their exposure to bright nocturnal light. The 7.5% of students who visited convenience stores almost every night had a delayed bedtime and a shorter time asleep. Multivariate logistic analyses revealed that this pattern was independently associated with total sleep time, daytime sleepiness, school-home commuting time, school grades and frequency of breakfast. Further studies are required to evaluate the contribution of other psychological and biological factors affecting circadian phase delay. However, our results showed the probable involvement of circadian rhythm alterations underlying the life-style difference observed in these students. Education in sleep including appropriate nocturnal use of convenience stores is required for effective school health-care practice.