Table 2.

Linear mixed effect model of factor associated with academic performance (stratified by outdoor time)

Variables	Total population		Outdoor time ≤ 2.3 h/day		Outdoor time > 2.3 h/day
	β (95%CI)	P	β (95%CI)	P	β (95%CI)	P
Outdoor time, hrs/day	6.494(4.148 ~ 8.840)	< 0.001	10.126(6.308 ~ 13.945)	< 0.001	1.022(-5.702 ~ 7.745)	0.766
Sleeping time, hrs/day	5.865(3.873 ~ 7.856)	< 0.001	5.690(3.430 ~ 7.949)	< 0.001	6.460(2.266 ~ 10.653)	0.003
Learning time out of school, hrs/day	-1.800(-2.700 ~ -0.900)	< 0.001	-1.375(-2.418 ~ -0.333)	0.010	-2.803(-4.603 ~ -1.003)	0.002
Gender, girls/boys	3.445(1.543 ~ 5.348)	< 0.001	3.289(1.078 ~ 5.500)	0.004	4.527(0.770 ~ 8.283)	0.018
Age	2.540(0.945 ~ 4.135)	0.002	2.509(0.649 ~ 4.369)	0.008	3.125(-0.022 ~ 6.273)	0.052
Father education
Junior high school and below	—		—		—
High school or vocational school	3.645(0.722 ~ 6.567)	0.015	3.407(-0.167 ~ 6.980)	0.062	4.036(-1.094 ~ 9.165)	0.123
Undergraduate or junior college	7.847(4.329 ~ 11.365)	< 0.001	8.343(4.180 ~ 12.506)	< 0.001	5.240(-1.468 ~ 11.949)	0.126
Master degree or above	9.122(2.003 ~ 16.241)	0.012	8.582(0.623 ~ 16.542)	0.035	6.938(-9.973 ~ 23.849)	0.421
Mother education
Junior high school and below	—		—		—
High school or vocational school	1.376(-1.510 ~ 4.261)	0.350	1.742(-1.729 ~ 5.213)	0.325	0.874(-4.383 ~ 6.131)	0.744
Undergraduate or junior college	5.932(2.525 ~ 9.338)	0.001	6.644(2.650 ~ 10.637)	0.001	-2.538(-24.239 ~ 19.164)	0.819
Master degree or above	9.776(0.614 ~ 18.937)	0.037	12.407(2.227 ~ 22.586)	0.017	2.987(-3.551 ~ 9.524)	0.37
Family income (RMB/month)
<  = 6000	—		—		—
6000–10,000	1.596(-0.909 ~ 4.100)	0.212	2.729(-0.264 ~ 5.723)	0.074	-0.633(-5.256 ~ 3.990)	0.788
10,000–20,000	4.359(1.544 ~ 7.175)	0.002*	4.935(1.626 ~ 8.244)	0.004*	4.037(-1.425 ~ 9.500)	0.147
> 20,000	3.987(0.385 ~ 7.590)	0.030*	4.830(0.760 ~ 8.900)	0.020*	2.051(-5.921 ~ 10.024)	0.614

No interaction was found between outdoor time and other possible factors: gender, P = 0.923; father education, P = 0.377; mother education, P = 0.178