Table 1 The general characteristics by gender of the participants

Mean ± SD

 BMI, kg/m²

17.84 ± 3.02

17.26 ± 2.58

18.45 ± 3.31

 < 0.001

 Outdoor time, hrs/day

2.02 ± 0.50

1.95 ± 0.48

2.10 ± 0.50

 < 0.001

 Sleep duration, hrs/day

9.53 ± 0.48

9.55 ± 0.48

9.50 ± 0.48

0.008

 Out-of-school learning time, hrs/day

2.43 ± 1.06

2.43 ± 1.06

2.43 ± 1.06

0.870

 Extracurricular class in summer vacation, hrs/week

3.26 ± 4.04

3.28 ± 4.04

3.23 ± 4.03

0.606

 Comprehensive academic performance

54.17 ± 27.32

55.77 ± 26.96

52.48 ± 27.60

0.001

  Chinese lesson

53.48 ± 25.91

56.19 ± 25.46

50.63 ± 26.07

 < 0.001

  Mathematics lessons

53.20 ± 25.62

52.56 ± 25.35

53.87 ± 25.88

0.161

  English lessons

53.65 ± 24.59

55.83 ± 23.80

51.34 ± 25.21

 < 0.001

n(%)

 Age

   ≤ 8 yrs

1249 (38.0)

630 (37.2)

619 (38.7)

0.509

  9 yrs

1598 (48.6)

838 (49.5)

760 (47.5)

   ≥ 10 yrs

444 (13.5)

224 (13.2)

220 (13.8)

 Father education

  Junior high school and below

819 (24.9)

403 (23.8)

416 (26.0)

0.543

  High school or vocational school

993 (30.2)

521 (30.8)

472 (29.5)

  Undergraduate or junior college

1282 (39.0)

665 (39.3)

617 (38.6)

  Master degree or above

90 (2.7)

46 (2.7)

44 (2.8)

 Mother education

  Junior high school and below

1024 (31.1)

496 (29.3)

528 (33.0)

0.113

  High school or vocational school

884 (26.9)

459 (27.1)

425 (26.6)

  Undergraduate or junior college

1235 (37.5)

657 (38.8)

578 (36.1)

  Master degree or above

47 (1.4)

27 (1.6)

20 (1.3)

 Family income (RMB/month)

   <  = 6000

835 (25.4)

415 (24.5)

420 (26.3)

0.567

  6000–10,000

1109 (33.7)

579 (34.2)

530 (33.1)

  10,000–20,000

883 (26.8)

446 (26.4)

437 (27.3)

   > 20,000

383 (11.6)

204 (12.1)

179 (11.2)