Study design and participants
This study was designed as a secondary data analysis using and interpreting data drawn from the third through thirteenth Korean Youth Risk Behavior Web-based Surveys (KYRBSs) completed across 7 years from 2013 to 2019. KYRBS was initiated by the Korean Ministry of Education, Ministry of Health and Welfare, and Centers for Disease Control and Prevention (KCDC) in 2005. The KYRBS is a cross-sectional, self-administered, anonymous, online survey that is conducted annually in a nationally representative sample of Korean adolescents aged 13–18 years. The understanding, reliability and validity of each question were investigated by the KCDC to qualify the surveys every year. It is known that the reliability estimates for the KYRBSs questionnaire show good validity [31]. The Institutional Review Board (IRB) of the KCDC approved this online statistical research during the entire survey every year. Survey participation excluded age-eligible respondents with absenteeism (school absence without permission), special educational needs (such as developmental disabilities), and dyslexia. In order to obtain a nationally representative sample, the KYRBS used the stratified three stage random cluster sampling method, which included stratification, sample allocation, and stratified cluster sampling, as is described. The study population was stratified by geographic region and school type to minimize sampling error. In sample allocation, 400 middle schools and 400 high schools were selected by proportional sampling to match the study population every year. In stratified cluster sampling, sample classes were selected by simple randomization sampling from the selected schools. All of the participants provided written informed consent (both directly and from their parents or legal guardians) for approval of survey access. Every student participated voluntarily and completed the survey by logging on to the survey’s Internet webpage at their school’s computer laboratory using a randomly assigned unique identification number. The participants were not able to skip to subsequent questions without providing proper answers to every question. The online system does not accept omitted responses. Data were considered to be missing if there was any logical error or if there was outlier. In the present study, we obtained the raw data that are publicly available from the online site of KYRBSs (https://www.cdc.go.kr/yhs/home.jsp). The reasons for drop-out or non-participation were not available.
Evaluation indices
1) Asthma
Participants were asked about their history of lifelong asthma using the following question: “Have you ever been diagnosed with asthma by a doctor at any point in your life?” The accepted responses were either “yes” or “no.” The disease severity, current symptoms, presence of management, and treatment modality were not assessed.
2) Demographic and socioeconomic variables
Information on demographic and socioeconomic characteristics (age, sex, school grade [middle school high school], residential type, perceived socioeconomic state [SES], and academic achievements) was evaluated. The type of residence was assessed as living with parents and without parents (such as living with relatives, living independently such as in a dormitory, or in an orphanage). The degree of SES and academic achievements were initially categorized as high, middle-high, middle, middle-low, and low. However, we categorized these as high (high or middle-high), middle (middle), or low (middle-low or low).
3) Health-related behavioral variables
The frequency of breakfast consumption was assessed with the question, “During the past 7 days, how many days did you eat breakfast?” with allowed responses of “never, 1, 2, 3, 4, 5, 6, or 7 days.” Those who indicated eating breakfast less than 2 days per week were called “breakfast skippers” according to the standards set in the survey. The frequency of vigorous exercise in the preceding 7 days, which was accompanied by very fast breathing or sweating, and by > 20 minutes of activity was also assessed. Vigorous exercise was classified into none, occasional (1–3 days), and frequent (more than 4 days per week). Participants were considered to be current smokers if they answered “more than 1 day over the past month” to the question “Do you smoke?” For alcohol consumption status, the current drinker was indicated by a reply of “more than 1 day over the past month” to the question ‘Do you drink alcohol?” in the standpoint of survey.
4) Emotional variables including suicidality
Psychological status was assessed using the following four variables: perceived health status, perceived level of happiness, perceived level of stress, and experience of depressive symptoms. Subjective healthiness was assessed with the following question: “What do you think about your health state?” The answer to this question was categorized as follows: healthy, average, and unhealthy. Perceived level of happiness was assessed based on a self-rating happiness scale and classified into happy, average, and unhappy. Perceived stress status was also assessed with the following question: “How often do you feel stress?” and was categorized as follows: often, sometimes and rarely. The following question was used to assess depression: “Within the last year, did you feel sad, blue, or depressed that caused cessation of your usual activities almost every day for two weeks or more?” This question was based on KYRBS and was answered with a binary response (yes or no). The measurement of suicidal ideation was also a binary dependent variable (yes or no) with the examining question being, “During the past 12 months, have you ever seriously thought of committing suicide?” Repetition of depressive mood and suicidal ideation were not assessed.
5) Sleep
The major independent variable for our study was sleep duration and WCUS. In the KYRBSs, self-reported wake up time and bedtime were determined on the basis of participants’ responses to the following questions separately for both weekdays and the weekend: (1) “What time did you usually go to bed and wake up during the weekdays (school days) over the last week?” and (2) “What time did you usually go to bed and wake up during the weekend over the last week?” The responses were provided separately for weekdays and the weekend as follows: (1) sleep time: ( ) o’clock ( ) minute AM/PM and (2) wake up time: ( ) o’clock ( ) minute AM/PM. The responses for sleep time on both weekdays and weekends were categorized as: ≤9:00, 9:00–10:00, 10:00–11:00, 11:00 PM-12:00AM, 12:00–1:00, 1:00–2:00, and ≥ 2:00 AM. Because wake up time was influenced by school attendance, the responses for wake up time were categorized as ≤ 5:00, 5–6:00, 6–7:00, 7:00–8:00, and ≥ 8:00 AM on weekdays, and ≤ 7:00, 7:00–8:00, 8:00–9:00, 9:00–10:00, 10:00–11:00, and ≥ 11:00 AM on weekends. We defined the subjects who went to sleep after 2 AM as night owls, and those who woke up before 7 AM as early larks based on the previous literature [3]. Sleep duration was also separately calculated for weekdays (sleep duration on school days) and weekends (sleep duration on school-free days) on the basis of their previous responses. The participants reporting sleep less than 2 h or more than 20 h for weekdays or weekends were excluded in this study. The average sleep duration was calculated using the following weighted mean value: (5 × weekday sleep duration + 2 × weekend sleep duration) ÷ 7 [3, 12], and categorized as: ≤ 5, 5–6, 6–7, 7–8, 8–9 and ≥ 9 h. Based on the recommendations from the National Sleep Foundation’s sleep time duration criteria [32], the reference sleep duration was defined by 7–8 hours per day in this study. WCUS was calculated as the average weekend sleep duration minus average weekday sleep duration [3, 12], and this was divided into four categories: no catch-up, < 1 hour, 1–2 hours, and ≥ 2 hours. The four categories were used because the latter two groups occupied the first and second largest proportions of WCUS and usually long WCUS was defined as sleeping at least 2 h longer on weekends compared to weekdays [33]. No catch-up has the same meaning as CUS ≤ 0 hour [34]. The levels of sleep satisfaction were assessed by the degree of recovery from fatigue by sleep according to the following question: “How satisfied are you with your sleep during the last week?” We re-categorized sleep quality into the following three groups: enough (plenty and enough), a little, and not enough (not enough and never enough).
Statistical Analyses
We conducted all of the statistical analyses using complex sample procedures of the Statistical Package for the Social Sciences (SPSS) software program version 21.0 (IBM Corp., Armonk, NY, USA). Because the KYRBS data were collected through a representative, stratified, and clustered sampling method (as a statistical representative of the general population), data from the survey were weighed based on the sample design. Descriptive statistics were used to depict the basic characteristics of the study population. The chi-square test for categorical variables and independent t-test for continuous variables were used appropriately to compare the general characteristics between subjects with and without asthma. Following the selection of significant covariates using univariable logistic regression analysis, multivariate logistic regression analysis was carried out to determine which sleep-related factors (e.g., sleep satisfaction, sleep duration, and WCUS) were independently contributing to the risk of suicidal ideation in adolescent with asthma: Model 1 adjusted for sex and grade; Model 2 adjusted for Model 1 variables + socioeconomic variables (residential type, SES, and academic achievement) and health-related behavioral variables (breakfast skipping, smoking, alcohol drinking, and vigorous exercise); and Model 3 adjusted for Model 2 variables + psychological variables (perceived status of health, perceived level of happiness, perceived frequency of stress, and experience of depressive symptoms). The results were expressed using adjusted odds ratios (OR) and 95% confidence intervals (CI). Statistical significance was indicated by p-values < 0.05 in all tests.