Table 1.

General characteristics of participants according to sleep duration

Factors	Total	Sleep duration					P values
		<6 hours	6 hours	7 hours	8 hours	9+ hours
Number
n	118 462	27 409	29 773	30 254	20 743	10 283
%	100	23.1	25.1	25.5	17.5	8.7
Mean age (year, SD)	15.0 (1.7)	16.1 (1.5)	15.5 (1.6)	14.7 (1.6)	13.9 (1.5)	14.0 (1.6)	<0.001*
Sex (%)							<0.001†
Male	50.2	41.8	47.7	52.3	57.1	59.5
Female	49.8	58.2	52.3	47.7	42.9	40.5
Obesity (%)							<0.001†
Underweight	6.1	5.9	6.2	6.0	6.2	6.9
Healthy	78.8	80.3	79.5	78.1	77.8	76.5
Overweight	11.3	10.8	10.8	11.8	11.8	11.8
Obese	3.8	3.0	3.5	4.1	4.2	4.8
Region (%)							<0.001†
Large city	44.7	49.4	46.6	43.3	40.6	39.0
Small city	47.3	44.5	46.4	48.1	49.8	50.1
Rural area	8.0	6.1	7.1	8.6	9.6	10.9
Stress (%)							<0.001†
Severe	8.9	13.7	9.3	7.1	5.7	7.1
Moderate	27.3	34.4	29.1	25.0	21.4	21.7
Mild	43.8	39.9	45.1	45.8	44.8	42.8
A little	16.6	10.1	14.3	18.8	23.0	21.9
No	3.3	1.9	2.3	3.3	5.2	6.6
Economic level (%)							<0.001†
Highest	8.1	7.3	6.9	7.9	9.5	11.1
Middle high	26.5	26.7	25.5	25.7	28.1	27.7
Middle	48.3	46.9	48.9	49.5	48.3	46.3
Middle low	14.0	15.2	15.2	13.9	11.8	11.9
Lowest	3.2	3.9	3.5	3.0	2.3	3.0
Educational level, father (%)							<0.001†
Unknown	19.7	12.8	16.0	21.9	26.6	28.5
Middle school	2.7	2.6	2.8	2.8	2.4	2.6
High school	29.2	28.8	30.5	30.3	27.5	26.9
College or over	48.4	55.8	50.8	45.0	43.6	42.0
Educational level, mother (%)							<0.001†
Unknown	18.9	12.0	15.2	21.0	25.8	28.1
Middle school	2.3	2.3	2.5	2.3	2.0	2.2
High school	37.1	39.1	39.3	37.1	33.7	31.8
College or over	41.7	46.7	43.0	39.6	38.5	37.9

*Linear regression analysis with complex sampling. Significance at p<0.05.

†χ² test with Rao-Scott correction. Significance at p<0.05.