24 hours society is rapidly advancing in JAPAN: convenience stores and so-called "Family- Restaurant", broad-casting radio and TV programs, and Internet programs, all of which we can enjoy 24-hours. Mobile phone is very convenient equipment for communication anywhere and anytime and now rapidly distributed not only to university students but also to young students in Japan. An epidemiological work was done on the effects of mobile phone usage by Japanese junior high school students (aged 13-15y), and university and physical therapist or nurse training school students (aged 18-25y) on morningnesseveningness preference and sleep habit. Everyday usage of mobile phone made the younger students more evening-typed, their wake-up timing delayed and their sleep hours shorter. Usage of mobile phone more than 20 min. per once usage made the elder students more evening-typed, their sleep latency longer, and them feel difficult to fall in sleep more frequently. Shifting to evening-typed partially caused by the usage of mobile phone can be possible to make psychiatry problems on the students which are supposed to be caused by internal de-synchronization of two circadian oscillators in humans. Usual usage of mobile phone in the evening or night might become effective "Electromagnetic and/or Psychological Zeitgeber(s)" for circadian oscillator driving sleep-wake cycle of Japanese students from chronobiological point of view.