subjects and procedures
A cross-sectional study was conducted in Shanghai, which is
located along the east coast of China with a total population of ~20.8
million in 2012 (State statistical bureau of the People’s Republic of
China, 2012). There are 17 districts in Shanghai, nine located in urban
areas and eight in suburban/rural areas. From these districts, four
located in urban areas and two located in suburban/rural areas were
randomly selected. For every district, two junior high schools and
two senior high schools were randomly selected. Study sample candidates were selected from the student population of these schools.
Among 5159 students eligible for the study, 4966 (96.25%) returned completed questionnaires. One hundred and sixty-five
children whose sleep duration was less than
−3 standard deviations from the mean (SD) and/or greater than
+3 SD were deleted.
The final sample consisted of 4801 children (49.1% boys vs 50.9%
girls). The mean age of the sample was 15.59 years (SD, 2.05; range,
11.0–20.0).
This study was conducted during November 2009. The research aims were explained to school principals and teachers in the
target schools. Permission was obtained to carry out the study, which
is the usual practice in China. The survey was implemented during
a regular health education class. In the class, an anonymous questionnaire was delivered to the students. Researchers explained the
study purpose to the students and emphasized that participation
was voluntary. Students were requested to fill out the questionnaires. All students who did not attend the class were
excluded from the survey.