Table 1.

Sample characteristics.

	Sample of n = 509 pupils
Gender, boys, %	52
Age at assessment in years, mean (SD)	13.10 (0.85)
Pubertal status category, %
Prepubertal	8
Early puberty	23
Midpubertal	37
Late puberty	29
Postpuberty	2
Educational track, %
Primary school	22
Pre-vocational secondary education	32
Pre-vocational/senior general secondary education	8
Senior general secondary education	10
Senior general secondary/pre-university education	16
Pre-university education	11
Caffeine, %
<1 per day	72
≥1–2 each day	17
≥2 each day	11
EDs, %
<1 per day	94
≥1 each day	6
Caffeine and EDs, %
<1 per day	65
≥1–2 each day	21
≥2 each day	14
Problems falling asleep, yes, %	23
Problems staying asleep, yes, %	6
Problems waking up, yes, %	23
Total of sleeping problems, mean (SD)	0.51 (0.79)
Hours of sleep, mean (SD)	8.83 (1.26)