Research design and research tools
This cross-sectional study is a secondary analysis of descriptive research using data from the 13th Korea Youth Risk Behavior Web-based Survey (KYRBS)  to investigate the effect of sleep habits on oral disease experience among Korean youth. The raw data from the Korean Youth Risk Behavior Web-based Survey was approved before use by the Centers for Disease Control and Prevention (CDC) in Korea . The study was conducted according to the relevant guidelines and regulations .
The KYRBS is a government-approved statistics survey (approval No. 117058) conducted every year since 2005 to identify health behaviors such as smoking, drinking, obesity, and eating habits among Korean youth. It is an anonymous self-filling web-based survey conducted for students from middle school to high school in Korea using a clustered, stratified, multistage probability sampling method .
A total of 54,776 out of 62,276 adolescents were selected, after excluding those who did not report the sleep duration details. The study participants were middle and high school students aged 12–18 years in Korean age (mean age: 15.0 years). During the KYRBS, each participant provided voluntary informed consent for participating in the survey .
Description of variables
Gender, school levels, academic grades, and economic status were selected as analytical variables. Gender was divided into “male” and “female.” First-, second-, and third-year middle schoolers were reclassified as “middle school students.” First-, second-, and third-year high schoolers were classified as “high school students.” For classifications according to scholarly achievement and economic status over the most recent 12 months, “top,” “mid-upper,” “mid,” “middle-low,” and “low” were reclassified into “top,” “middle,” and “low” .
Oral health behavior
Frequency of tooth brushing The question asked was, “How many times did you brush your teeth yesterday?” A reply of “0” was excluded because there were no respondents, and only “1” and “2 or more” were classified.
Flossing habit The question asked was, “In addition to toothpaste and toothbrush, mark all the items you currently use for your oral health.” The answers were classified as “Used” and “Not used.” .
Sleep types Sleep was divided into two types depending on individual bedtime. We classified early bedtime (before 1 a.m.) as type A and late bedtime (after 1 a.m.) as type B. This classification was based on a previous study  and the characteristics of our research data. Sleep duration Using the Korean Youth Risk Behavior Web-based Survey data, the sleeping period was discerned as a response to this question: “During the last 7 days, what time did you go to bed, and what time did you wake up from sleep?” The answers were to be written in hours and minutes separately for weekdays (Monday to Friday) and weekends (Saturday and Sunday). Sleep duration was divided into weekday and weekend in the raw data . However, only “weekday” data were used in this study to help discern adolescent daily life. We calculated sleep duration based on the methodology followed in previous studies  and guidelines  for research data analysis. As the standard for adequate sleep for adolescents provided by the U.S. National Sleep Foundation is 8–10 h , the frequency distribution of sleep duration was classified as “less than 6 h,” “more than 6 h but less than 7 h,” “more than 7 h but less than 8 h,” or “more than 8 h.”
Sleep quality For the question, “Do you think that the amount of sleep you had over the past 7 days is sufficient for fatigue recovery” from the Korean Youth Risk Behavior Web-based Survey data, the responses “quite enough,” “enough,” “moderate,” “insufficient,” and “very insufficient” were reclassified into “enough,” “moderate,” and “insufficient” .
Experience of symptoms of oral disease
Symptoms related to oral disease were selected from the Korean Youth Risk Behavior Web-based Survey. Individuals were questioned about oral symptoms experienced during the last 12 months. Five items were used: “toothache or throbbing,” “toothache on chewing,” “gingival pain and bleeding,” “pain in the tongue and buccal mucosa,” and “halitosis” .
Following the KYRBS data analysis guidelines, a complex sample data analysis was conducted using corrections for strata, cluster, weight, and a finite population. Using SPSS statistics-version 25.0, weights were applied to ensure appropriate representation of Korean youth in the web-based survey complex samples of youth risk behaviors .
Crossover analysis-logistic regression was used to examine the relation of symptom experience. For logistic regression analysis, adjusted variables were gender, school level, grade, economic status, tooth brushing frequency, flossing habits, sleep types, sleep duration, and sleep quality. Statistical significance was set at p < 0.05.