Table 4.

Correlation Coefficients Between Sleep-Wake Variables and Students' Risk Factors

Variable	School night bedtime	School night total sleep time	Weekend oversleep	Sleep Quality Index scorea	Epworth Sleepiness Scale scorea
Age (yr)	0.23**	−0.31**	0.15**	0.12**	0.15**
Sex (0 = girls; 1 = boys)	−0.06**	0.06*	−0.19**	−0.09**	−0.11**
Parents martial status
(0 = married or cohabiting, 1 = divorced or widowed)	−0.01	−0.05*	0.05	0.05*	0.05
School band (1 to 3)b	−0.15**	0.14**	−0.05*	0.04	−0.02
Grade level (7 to 12)	0.32**	−0.31**	0.16**	0.07**	0.16**
Traveling time to school (min)	−0.09**	−0.20**	0.17**	0.07**	0.11**
Part-time work (hr per week)	0.10**	−0.07**	0.008	0.07**	0.10**
Overweight (0 = no, 1 = yes)c	−0.08**	0.007	−0.04	−0.02	−0.08**
Had smoking habit (0 = no, 1 = yes)	0.11**	−0.11**	0.10**	0.07**	0.08**
Alcohol used	0.12**	−0.16**	0.12**	0.16**	0.17**
Coffee used	0.09**	−0.19**	0.08**	0.08**	0.14**
Physical exercisee	−0.07**	0.07**	−0.11**	−0.08**	−0.07**
Perceived Stress Scale scoref	0.12**	−0.14**	0.14**	0.36**	0.26**
M/E scale scoreg	−0.43**	0.26**	−0.34**	−0.25**	−0.33**

^aHigher scores indicate greater severity of sleep disturbance and daytime sleepiness. ^bLower band indicates higher school academic ranking. ^cInternational Task Force on Obesity overweight reference, sex-age-specific BMI cut-offs that correspond to BMI = 25 at age 18 yrs. ^dSelf-report of alcohol and coffee use: 0 = never, 1 = rarely, 2 = sometimes, 3 = almost always, 4 = often. ^eSelf-report of the frequency of physical exercise per week: 0 = nil, 1 = 1 to 2 times, 2 = 3 to 4 times, 3 = 4 to 5 times, 4 = 6 times or more. ^fHigher scores indicate greater perceived stress. ^gM/E scale, Morningness/Eveningness scale for adolescents. Lower M/E scale scores indicate more extreme evening-type. *P < 0.05; **P < 0.01.