Table 1 Characteristics of participants by grade

Sex (boy)

50.1

49.6

0.83

 < 6:30 a.m.

52.8

48.6

0.07

 6:30 to < 7:00

44.2

46.6

 ≥ 7:00

3.0

4.8

Skipping breakfast (yes)

4.5

8.7

< 0.001

Screen time on week day (≥ 2 h)

34.5

43.2

< 0.001

 Very often

28.3

28.7

< 0.001

 Often

48.8

39.7

 Rarely

22.9

31.7

 < 10 p.m.

88.6

53.7

< 0.001

 10 to < 10:30

9.2

31.2

 10:30 or later

2.2

15.2

Daytime sleepiness (yes)

12.8

20.9

< 0.001

Obesity (yes)

8.2

7.8

0.73

Cram school (yes)

23.0

27.0

0.06

Study duration at home (< 1 h)

74.8

50.3

< 0.001

Paternal smoke (yes)

42.1

46.2

0.09

Maternal smoke (yes)

10.2

11.7

0.32

 High

28.3

26.9

0.73

 Middle

47.8

47.9

 Low

23.8

25.2

Subjective academic performance

0.20

 Well or relatively well

82.4

81.6

 Neither

11.9

14.1

 Relatively poor or poor

5.7

4.3