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).