Table 1.

Overview of used variables.

Variable	Question	Response Categories	Data Processing	Reference
Socio-economic status (SES)	Family Affluence Scale III consisting of six items: number of cars family owns, having own bedroom, number of computers family owns, number of bathrooms at home, having a dishwasher at home, and number of travels for a vacation last year.	Number of cars family owns: None (0), one (1), two or more (2) Own bedroom: No (0), yes (1) Number of computers: None (0), one (1), two (2), more than two (3) Number of bathrooms: None (0), one (1), two (2), more than two (3) Dishwasher: No (0), yes (1) Family holidays: Not at all (0), once (1), twice (2), more than twice (3)	A sum score was calculated (range 0–9) and transformed into a continuous variable by performing a ridit transformation (range 0–1). In that way, the Family Affluence Scale indicates the relative material position within society (i.e. country). The scale also takes population heterogeneity for age and gender into account. Higher values reflect higher SES.	Material affluence was used as a measure of SES [23], using the Family Affluence Scale III [24]. The sum score of the six items showed high test–retest reliability (r = 0.90) and consistency between child and parent report (r = 0.80)
School pressure	“How pressured do you feel by the schoolwork you have to do?”	Not at all or a little (1), some (2), a lot (3)
Physical activity	“Over the past 7 days, on how many days were you physically active for a total of at least 60 min per day?”	None (0) to seven (7) days		Biddle et al. [25] report a reasonable validity and moderate reliability for this index. Prochaska et al. [26] validated this measure for use with children and adolescents.
Screen time	“How many h a day, in your free time, do you usually spend watching television (TV), videos (including YouTube or similar services), DVDs, and other entertainment on a screen?”; “How many h a day, in your free time, do you usually spend playing games on a computer, games console, tablet (like iPad), smartphone or other electronic device (not including moving or fitness games)?”; and “How many h a day, in your free time, do you usually spend using electronic devices such as computers, tablets (like iPad) or smart phones for other purposes, for example, homework, emailing, tweeting, Facebook, chatting, surfing the Internet?”	None at all (1), “half an hour a day” (2), “1 h a day” (3), “2 h a day” (4), “3 h a day” (5), “4 h a day” (6), “5 h a day” (7), “6 h a day” (8), “7 h or more a day” (9)	Assessed separately for week- and weekend days. Response categories were transformed to number of h using the midpoint method. To represent the overall amount of screen time per day, the three screen time behaviours were summed for week- and weekend days separately. Finally, the mean screen time on weekdays and weekends together was calculated ((5 × weekdays) + (2 × weekend days)/7).	Rey-López JP et al (2010) [27]
Psychological symptoms	“In the last 6 months: how often have you had the following… (1) feeling low, (2) irritability or bad temper, and (3) feeling nervous?”	Five: “about every day” (1), “more than once a week” (2), “about every week” (3), “about every month” (4), and “rarely or never” (5)	Response categories were reversely scored to facilitate interpretation of the results.	Three psychological symptoms of the Health Behaviour in School-Aged Children (HBSC) Symptom Check List (HBSC-SCL) were used [28]. The HBSC-SCL consists of four questions on somatic and four questions on psychological symptom load. The internal validity of the psychological subscale was good (α = 0.78) [28]. When leaving out the symptom of sleep onset difficulties from the psychological symptom load generated by the HBSC-SCL, the internal consistency of the psychological dimension slightly improved (Cronbach’s alpha increased from 0.78 to 0.79).
Sleep onset difficulties	“In the last 6 months: how often have you had the following… difficulties in getting to sleep?”	Five: “about every day” (1), “more than once a week” (2), “about every week” (3), “about every month” (4), and “rarely or never” (5)	Response categories were reversely scored to facilitate interpretation of the results.	This is the fourth psychological symptom of the HBSC-SCL [28].
Sleep duration week	“When do you usually go to bed if you have to go to school the next morning?”	Eleven: ranging from “no later than 21.00” to “02.00 or later”	Response categories were transformed to h using the midpoint method.	Wolfson et al. found that school and weekend nights survey variables were significantly correlated with actigraphy and diary variables [29].
	“When do you usually wake up on school mornings?”	Seven: ranging from no later than 05.00” to “08.00 or later”	Sleep duration was calculated by subtracting the reported bedtime from the wake-up time.
Sleep duration weekend	“When do you usually go to bed at weekends or during holidays?”	Fifteen: ranging from “no later than 21.00” to “04.00 or later”
	“When do you usually wake up at weekends or during holidays?”	Fifteen: ranging from “no later than 05.00” to “14.00 or later”