Specifying the major factors, which induce the extreme delayed sleep and wake pattern frequently observed in Japanese people.

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According to the survey conducted once every 5 years by NHK (Japanese corporation for public broadcasting), Japanese bedtime has became delayed about 1.5 hour during these 40 years and accordingly the total sleep time decreased for about 1 hour. Compared these conditions with other developed countries, i.e., European and North American countries, Japanese show an extremely delayed bedtime and reduced total sleep time. Why this nocturnally shifted lifestyle was developed among Japanese has not yet explained scientifically, many socio-cultural interpretations was proposed to"explain"the phenomenon, though. TV watching and cellular phone was pointed out as key factors ; however, those data necessarily prove its causality.We investigated these candidate factors, i.e., mass media and photo environment with university students and elderly persons as the subjects. They were restricted the total hour of watching TV in the second week and their lifestyle was compared with the baseline week. In elderly group, there are no significant differences between the two conditions. On the contrary, in the students, they showed delayed rise time as well as delayed bedtime in the TV restricted week. The students also showed the reduced activity (measured by a watch-type activity monitor) in the TV restricted week. Moreover, the students who showed extremely delayed lifestyle exposed themselves in an intense photo environment in the afternoon period as well as in the midnight.These results indicate that the same factor plays different rolls in different age groups, and in university students, who has no strong social constraints, TV has a roll of waking them up in early hours and making them active during daytime, as well as of delaying their bedtime. To make effective interventions against the delayed lifestyle, it is crucial to control those key factors and to take the characteristics of the subjects into consideration.