Study population and design
With the consent of the President of the Association of High School Principals and the prefectural Education Bureau of a certain prefecture in Japan, we sent requests for participation to the principals of all 54 day-boarding high schools within the above 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 survey form to be used in the study. We stated 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 day-boarding high schools.
The survey procedure was as follows: (1) the teachers distributed the following three items: an explanatory document, a self-administered questionnaire form, 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 questionnaire forms were first unsealed and opened when they were to be used for data input at the assigned research facility. The survey period was from June to December 2016.
The survey forms collected information on participant demographic characteristics, sleep disturbance, and IA.
Information was collected on the name of the school, grade, and name and sex of the student. By school names, students were further grouped on the basis of 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 questions on emotions and perceptions were measured by depressed mood (mental health) and school-life satisfaction.
We adopted the measure of depressed moods (mental health) from 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 with your school life? Or are you dissatisfied with it?”
Measurement of sleep disturbance: Japanese version of the Pittsburgh Sleep Inventory
Sleep disturbance was evaluated by the Japanese version of the Pittsburgh Sleep Quality Index (J-PSQI) [35-37]. On the basis of 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
IA was evaluated using the Young Diagnostic Questionnaire (YDQ) [3, 38-44]. For the present study, we used the Japanese version of the YDQ (J-YDQ) that has been used in previous studies . The J-YDQ is an evaluation tool composed of 8 questions, which are rated 1 point for “yes” and 0 point for “no,” with the total score ranging from 0 to 8 points. The participants where then grouped into three categories: “IA,” when 5 of the 8 YDQ items were present; “at-risk,” when either 3 or 4 YDQ items were reported; and “non-IA,” when less than 2 YDQ items were positive. [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 either directly from the students, if their supervising teacher confirmed that their judgment was acceptable, or from their parents, if the supervising teacher thought that the parents' consent was necessary. 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 forms were stored securely, and data were entered into a password-protected database. Data were anonymized prior to the analysis by deleting all personal identifiers. The Faculty of Medicine, Oita University Ethics Committee approved the study (approval no. 932).
Students who did not completely fill out the J-PSQI and the YDQ were excluded from the analysis. First, distribution by sex (boys or girls) was plotted for the J-PSQI and YDQ scores. Second, prevalence rates for different categories of sleep disturbance and IA were calculated separately for each sex by using the chi-square test. Furthermore, for each of the three YDQ categories, prevalence rates for sleep disturbance were calculated for each sex separately. Finally, multiple logistic regression analysis was conducted to investigate the relationship between IA (explanatory variable) and sleep disturbance (dependent variable). The type of school, school-commute time, sports and club time, outside-class study time, television-viewing time, skipped meals, depressed mood (mental health), and school-life satisfaction rates were used as adjusted variables. The Statistical Package for Social Sciences version 22 (SPSS, IBM Corp. NY, USA) for Windows was used for all statistical analyses. A p-value <0.05 was considered statistically significant.