Table 1. Characteristics of participants by sleep duration category (N = 66,817).

Characteristic	Sleep Duration, hours/day
	<5.0			5.0 ~ 6.9			7.0 ~ 8.9			≥9.0
	N	%wt	Mean (SD)	N	%wt	Mean (SD)	N	%wt	Mean (SD)	N	%wt	Mean (SD)
Female sex	487	47.0		14,263	53.5		20,993	50.1		983	37.2
Age, years			16.2 (1.5)			16.4 (1.4)			15.5 (1.7)			14.9 (1.5)
Grade
Junior high school§	418	42.5		8,530	33.7		23,150	58.1		1,832	74.7
High school & vocational school¶	533	57.5		16,053	66.3		15,678	41.9		623	26.3
Parental smoking	496	54.4		12,420	52.3		19,340	51.8		1,237	53.0
Parents highest education at college level
Both had college degree	82	8.7		1,029	4.4		1,809	4.8		115	5.1
Only one of them had college degree	101	11.0		1,855	7.8		2,821	7.5		140	5.9
None of them had college degree	756	80.3		21,433	87.8		33,669	87.7		2,158	89.0
Single family	87	9.6		1,302	5.4		1,617	4.3		100	4.3
Sleep duration, hours			4.5 (0.7)			6.7 (0.4)			8.0 (0.5)			9.8 (0.6)
Reported BMI&			21.1 (6.3)			20.3 (5.1)			20.0 (5.3)			20.3 (5.9)
Corrected BMI&,$			22.1 (6.6)			21.5 (5.5)			21.7 (6.2)			21.7 (6.2)
Overweight	179	23.6		3,498	16.4		6,430	18.8		559	26.8
Obesity	94	16.0		1,602	11.2		3,081	13.2		302	18.8
Physical activity	308	32.3		7,757	31.3		9,552	24.7		528	27.2
Computer and video games ≥2 hours/day	309	34.0		3,218	13.8		3,521	9.6		187	8.3
Home work ≥3 hours/day	362	38.3		7,695	31.1		5,191	13.6		205	8.5
Smoking	156	18.0		1,209	5.5		1,405	4.1		129	6.2
Alcohol	360	38.9		4,880	21.1		6,168	16.9		387	16.9
Depression	616	64.7		14,042	57.2		15,708	40.8		709	29.5
Anxiety	522	55.5		11,431	46.8		14,240	37.0		706	29.2

Abrreviations: BMI; Bady Mass Index; SD: Standard Deviation.

^§Grades 7–9.

^¶Grades 10–12; ^&Weight (kg)/height (m2); ^$BMI after correction.