The present study investigated the relationship between school bullying and sleep pattern or nocturnal cellular phone usage in the Japanese adolescents (7th–9th grade). Bullying behavior was significantly associated with irregular sleep and e-mail exchange/calling after lights-out after controlling sex, age, survey area, the number of parents living together, drinking alcohol, usage of recreational drugs, and domestic violence from adults. In addition, school size was also significantly associated with the increased risk of bullying in junior high school students (7th–9th grade). To our knowledge, this is the first study to observe the significant effect of nocturnal lifestyle such as sleep pattern and cellular phone usage on school bullying.
The association between irregular bedtime and the bullying status suggests that problems in sleep pattern may be a causing or moderating factor for bullying behavior. In junior high school students, irregular bedtime was significantly associated with both being pure bully and bully-victim. The association was not statistically significant in senior high school students, while the trend was the same as in junior high school students when the odds ratios were referred. The statistical difference between junior and senior highs school may be explicable by the statistical power because the number of subjects and the prevalence of bullying were smaller in senior high school than in junior high school. Adolescence is a developmental period accompanied with changes in sleep characteristics 
. It has been suggested that the way of sleeping critically affect daytime functioning, including thinking, feeling, and behavior, in adolescence 
. Wake-up and fall-asleep rhythm is spontaneously accompanied with rise and decline rhythm of the body temperature 
. When this reciprocal interaction is impaired, physical or mood disturbance occurs and may work as a biological mechanism for the critical effect of the problematic sleep pattern on bullying behavior 
. The present result may warrant the importance of school-based interventions to improve nocturnal behaviors as a method to prevent school bullying.
E–mail exchange/calling after lights-out was remarkably associated with pure bullies in both junior and senior high school students, and also with bully-victims in senior high school students. The associations suggest that having a cellular phone and e-mail exchange/calling after lights-out might facilitate bullying. For instance, a cellular phone may work as a major communication method to conduct psychological bullying including excluding, isolating, and gossiping. It may be necessary to increase awareness of electronic bullying among students, parents, and school personnel for prevention. Nocturnal sleep duration was also associated with the bullying status, including being pure victim and being bully-victim, especially in junior high school students. This could be due to insomnia or hypersomnia as a symptom of depression resulting from victimization.
It is of note that a significant association was observed between the bullying status and school size. Large school size was significantly associated with the bully-victim status. Bowes et al. 
showed that large school size was associated with an increased risk for victimization in seven-year-old children. The possible explanation for the effect of school size is that escaping from teachers' supervision might be relatively easier for students in large-size schools. This finding may be valuable from a viewpoint of developing preventive measures, while further research is needed to confirm the result and elucidate the mechanism.
Prevalence of the bullying status was basically comparable with previous Japanese studies for junior high school students (12%, 6%, and 5% in the present study vs. 9%, 6%, and 5%, and 14%, 7%, and 5% in 1,994 and 1,248 students in the previous studies 
, for pure bullies, pure victims, and bully-victims, respectively). No previous data was available for senior high school students in Japan. GHQ-12 score, which could represent increased internalizing problems, i.e. anxiety and depression 
, was elevated in pure victims. Drinking alcohol and recreational drug usage were in contrast, increased in pure bullies, which might correspond to externalizing problems, such as aggression and antisocial behaviors 
. The risk for these problems was the highest in bully-victims, which supports the previous finding that they are a distinctive group of the most troubled adolescents 
Caution may however be needed in interpretation of the present data. First, this is a cross-sectional survey, and therefore there is a possibility that the problematic sleep pattern and/or e-mail exchange/calling after lights-out might be a result, not a cause, of the bullying status. A longitudinal study is needed for the answer to the question. Second, data on socioeconomic status was not available and therefore not controlled. Third, answers from absent students were not available, while bullying and victimization could be more prevalent among the frequent or long-term absentees. Lastly, we used simple self-report questions with a relatively small number of items to evaluate the bullying and victimization. Subjects could over- or under-report on these topics. More details of sleep pattern and problematic cellular phone usage, such as differences between weekdays and weekends, may be examined in future studies.
In conclusion, the present study suggested that irregular bedtime and cellular phone usage after lights-out might be risk factors for being involved in school bullying in adolescents. Progressive trend towards nocturnal lifestyle and increasing usage of cellular phone might impair the well-being of adolescents. School-based interventions for lifestyle including sleep pattern and cellular phone usage may be encouraged to reduce school bullying.