2.3 | Independent variables of main interest: sleep-related factors
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.5 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 average sleep duration was calculated using the following weighted mean value: (5 × weekday sleep duration + 2 × weekend sleep duration)\(\div\)7,5,15 and categorized as: ≤ 5, 5-6, 6-7, 7-8, 8-9 and ≥9h. Based on the recommendations from the National Sleep Foundation’s sleep time duration criteria,20 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,5,15 and this was divided into four categories: ≤0 hour, 0–1 hour, 1–2 hours, and ≥ 2 hours. Non-CUS has the same meaning as CUS ≤ 0 hour.21 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).