Study population and design
After obtaining the consent of the President of the Association of High School Principals and the prefectural Education Bureau of one prefecture in Japan, we sent requests for participation to the principals of all 54 daytime high schools within the prefecture and sent the following documents via the postal service to each principal: (1) letter requesting cooperation; (2) planning document containing the study purpose and method; and (3) the questionnaire to be used in the study. We specified that a self-administered questionnaire form would be used in the survey, with assured protection of respondent privacy. A total of 10,405 students were registered at the 54 daytime high schools.
The survey procedure was as follows: (1) the teachers distributed the following three items: an explanatory document, a self-administered questionnaire, and an envelope; (2) after filling in their responses in the questionnaire form, the surveyed students placed the completed questionnaire form in the provided collection envelope and sealed the envelope; (3) the teachers collected the sealed envelopes; and (4) the envelopes containing the self-administered questionnaires were not unsealed and opened until they were used for data entry at the research facility. The survey period was from June to December 2016.
The questionnaire collected information on participant demographic characteristics, sleep disturbance, and IA.
Data were collected on the name of the school, grade, and name and gender of the student. After recording the school names, participants were classified according to whether they were attending a public school or a private school. Questions on daily-life habits included school-commute time, time spent engaging in school sports or clubs, time spent on study outside school hours, television-viewing time, and skipped meals. These questions were similar to those used in previous studies among adolescents [10, 31-33] (Appendix A). The items on emotions and perceptions were measured by assessing depressed mood and school-life satisfaction.
We adopted the measure of depressed mood used in previous studies [31, 33]. The question was: "Over the past 30 days, did you have feelings of heaviness or depression more than usual?" We measured school-life satisfaction using a 2013 survey conducted by the Cabinet Office on the attitudes of young people in Japan and other countries . The question was: "Are you satisfied or dissatisfied with your school life?"
Measurement of sleep disturbance: Japanese version of the Pittsburgh Sleep Inventory
Sleep disturbance was evaluated using the Japanese version of the PSQI (J-PSQI) [35-37]. Based on previous studies, scores ≥ 5.5 points on the J-PSQI were considered indicative of sleep disturbance [35-37].
Measurement of IA: Japanese version of the Young Diagnostic Questionnaire
We measured IA using the Young Diagnostic Questionnaire (YDQ) [3, 38-44]. We used the Japanese version of the YDQ (J-YDQ) which has been used in previous studies . The J-YDQ is an evaluation tool composed of eight questions, which are scored as 1 point for "yes" and 0 points for "no," with the total score ranging from 0 to 8 points. The participants were grouped into three categories: "IA," if they scored 5–8 points, "at-risk," if they scored 3–4 points, and "no IA," if they scored 0–2 points [25, 33, 39, 43, 45, 46].
The participation of students in the present study was voluntary. As our cohort included 15- to 16-year-old adolescents, we obtained written informed consent directly from the students or their parents when their supervising teacher confirmed that their judgment was acceptable or he/she thought that the parents' consent was necessary, respectively. The following statements were included in the consent document distributed to students and their families: (1) the survey was part of an epidemiological study and involved neither an evaluation for school grading nor any type of punishment; (2) students were free to cooperate in the survey, and failure to cooperate would not incur any disadvantage; (3) the school teachers would not view the responses provided; and (4) respondent privacy would be strictly protected. The study questionnaires were stored securely, and data were entered into a password-protected database. Data were anonymized before the analysis by deleting all personal identifiers. The Faculty of Medicine of the Oita University Ethics Committee approved the study (approval no. 932).
Students who did not complete the J-PSQI and the J-YDQ were excluded from the analysis. All analyses were stratified by gender. First, we plotted for the J-PSQI and J-YDQ score distributions. Second, participants were categorized as having a sleep disturbance of not according to their J-PSQI score, and categorized as not having IA, being at risk of IA, or having IA according to their J-YDQ Score. We calculated the prevalence of sleep disturbance according to IA status and determined whether there was a significant association between internet addiction and sleep disturbance using the chi-square test. Third, we conducted multiple logistic regression to measure the association between IA (as an explanatory variable) and sleep disturbance (as the dependent variable). The type of school, school-commute time, sports and club time, outside-class study time, television-viewing time, skipped meals, depressed mood, and school-life satisfaction were used as adjustment variables. The Statistical Package for Social Sciences Version 22 (SPSS, IBM Corp. NY, USA) for Windows was used for all statistical analyses. P-values <0.05 were considered statistically significant.