Participants
This study performed a secondary data analysis based on the 15th 2019 Korean Youth Risk Behavior Web-based survey (2019, KYRBS) data. KYRBS is conducted by the Korean Disease Control and Prevention Agency (KDCA) annually to evaluate the physical and mental health status of Korean middle and high school students [21]. KYRBS adopted a multi-stage cluster sampling design to obtain a representative sample of Korean adolescents, dividing 17 provinces nationwide into 44 regions according to the size of the city and selecting middle school (grades 7–9) and high school (grades 10–12) students among 400 schools. KYRBS is an anonymous self-response online survey, and the selected students participated using a computer in their school’s computer room under the supervision of trained teachers. Students could decide whether to participate after receiving a detailed explanation of the survey’s purpose and process from the trained teacher, and written informed consent was obtained from the participants. Information on the detailed research design and research methods of KYRBS can be found in a previous paper [21]. In the 2019 KYRBS, a total of 60,100 people from 800 schools (400 middle schools and 400 high schools) were selected as subjects, of which 57,303 (95.3%) who agreed to participate in the survey were included. Since KYRBS is based on an online survey system, there were no non-response items in the original data, but logical errors and outliers were treated as missing values. In this study, the data file for the SPSS program provided by KDCA was used for analyses.
Assessment
Demographics
We analyzed the individual demographic data including gender, age, size of the city, school type, grade, educational background, perceived academic performance, perceived household economic status, type of residence, and parents’ nationality. The size of the city was divided into big, small to medium, and small in consideration of geographical accessibility, number of schools and population, and living environment. Perceived academic performance and perceived household economic status were classified into five categories: upper, upper middle, middle, lower middle, and lower. The type of residence was evaluated as (1) living in a parent’s house, (2) living in a relative’s house, (3) lodging or living alone, (4) living in a dormitory, and (5) living in a childcare facility. Information on the family—including parental nationality—was obtained when the subjects agreed to answer the question. In this study, we classified nationalities as either Korean or other nationalities.
Health risk behavior and health status
The frequency of breakfast during the week, the number of days of physical activity for more than 60 minutes a day during the week, and the weekday/weekend sitting time for studying and using internet were evaluated. As for the question on physical activity, we assessed the number of weekdays in which their heart rate was higher than usual or they did 60 minutes or more of breathtaking physical activity, regardless of the type of physical activity. The participants were asked to report the average weekday/weekend sitting time autonomously. Considering that the subjects spent around six hours at school, weekday sitting time for studying was classified as < 240 min, 240–359 min, 360–479 min, 480 –599, min, 600–719 min, ≥ 720 min, and weekend sitting time for studying was divided into none, 1–119 min, 120–239 min, 240–359 min, 360–479 min, and ≥ 480 min. Internet use time was classified into none, 1–60 min, 61–120 min, 121–180 min, ≥ 180 min. The subjective perception of their own health status was answered with five responses: “very healthy,” “healthy,” “so so,” “unhealthy,” and “very unhealthy.” In addition, the subjective perception of body shape was evaluated as follows: “very skinny,” “skinny,” “so so,” “obese,” and “very obese.” Health risk behaviors such as smoking, alcohol use, habitual drug/substance use, and sexual experience were also evaluated. Participants also answered whether they had received hospital treatment for violent incidents within one year; this was classified as “yes” or “no” for the regression analysis.
Mental status
The perceived stress level and subjective experience of depressive episodes are included in this study. The former was classified into five responses: “very much,” “a lot,” “a little,” “not much,” and “not at all,” and the latter was evaluated by answering the question “Did you ever feel sad or desperate strong enough to stop your daily life for 2 weeks in the last 12 months?”
Statistical analyses
In this study, students who provided insufficient answers were excluded; in the end, a total of 46,206 subjects were included in the analysis. All statistical analyses were performed using the SPSS 25.0 software (IBM corp, Armonk, NY, 2019), and p-values of less than 0.05 were considered statistically significant. The variables, including demographics, health risk behaviors, health risk behaviors, weekend sleep time, and physical/mental status, were compared by grade groups using a one-way analysis of variance (ANOVA) and chi-square test. ANOVA, chi-square test, and logistic regression were used to compare the incidence of depressive episodes and other individual variables between groups according to weekday sleep time. Logistic regression analyses including all variables were performed to confirm the relationship between individual variables and depressive episodes. Binary regression analysis with the depressive episode as the dependent variable was performed, followed by regression analysis between statistically significant variables and the depressive episode to evaluate the effect of weekday and weekend sleep time on depressive episodes. The results of logistic regression have been reported as unadjusted and adjusted odds ratios with 95% confidence intervals.
Ethics statement
KYRBS has been conducted by Ministry of Education, Ministry of Health and Welfare, and KDCA as national approved statistical data [21]. The raw data used in this study were approved for use through the KDCA website, and the provided raw data were collected with a unique number that cannot be identified without the subject’s personal information, ensuring the subject’s anonymity. The protocol of this study for secondary analysis was approved by the Institutional Review Board of the Korea University Medical Center, Ansan Hospital, Gyeonggi-do, Korea (No. 2020AS0309).