Sleep habits
Time of going to bed
About 14.2% of the subjects went to bed before 10 pm and 19.6% went to bed after midnight. The time of going to bed (TGT) was associated with grade level (Chi-squared = 37.03, df = 12, p < 0.001). Adolescents in grade 11 and 12 went to bed later than those who were in grades 9 and 10. In addition, TGT was associated with gender (Chi-squared = 30.05, df = 4, p < 0.001) and girls usually went to bed earlier than boys.
Sleep latency
The question on sleep latency of students showed the following distribution of sleep latency categories: less than 6 min in 12.2%; > 5 and < 11 min in 35.8%; > 10 and < 31 min in 35.4%; > 30 min in 16.7%. Sleep latency was not associated with gender (Chi-squared = 7.3, df = 3, p = 0.06) or grade level (Chi-squared = 2.24, df = 9, p = 0.98).
Waking at night
The question about waking at night showed the following distribution categories: never in 44.6%, 1 to 2 times in 47.5%, 3 to 4 times in 5.9%, 5 to 6 times 1.5%, and more than 6 times in 0.6%. It was associated with gender and the frequency in girls was higher than that in boys (Chi-squared = 19.9, df = 4, p < 0.001). In addition, it was associated with grade level (Chi-squared = 21.9, df = 12, p < 0.03).
Morning waking time
The mean morning waking time was 07.17 am. The time of waking in the morning in girls was 07.14 (SD = 1.3) and in boys it was 07.19 (SD = 1.3). There was no difference between genders (t = 0.66, df = 1,257, p = 0.5). The mean waking time was different between grade level (F3,1255 = 17.2, p < 0.05). The mean morning waking time for 12th grade adolescents was 07.17 (SD = 1.2) while it was 06.57 (SD = 1.3) for the 9th grade subjects.
Sleep duration
The mean duration of night sleep was 7.7 h (SD = 1.3). The mean duration was 7.70 h (SD = 1.4) in girls and 7.79 h (SD = 1.3) in boys. There was no difference between girls and boys (t = 1.29, df = 1,329, p = 0.19). The mean durations at different grade levels (grades 9, 10, 11, 12) were 7.63, 7.86, 7.83, 7.65 h, respectively (F3,1327 = 4.7, p = 0.057). However, sleep duration decreased from grades 10 to 12.
Sleep-related symptoms
Sleep quality
Sleep quality at night showed the following distribution categories: excellent in 25.6%, good in 41.9%, satisfactory in 25.1%, poor in 5.6%, and very poor in 1.8%. It was not associated with grade level (Chi-squared = 4.2, df = 12, p = 0.97). The quality of sleep was not correlated with age. About 10% of the girls and 4.6% of the boys perceived their quality of sleep as being bad (Chi-squared = 14.9, df = 1, p < 0.001).
Daytime sleepiness
About 42% of the students reported that they never or rarely experienced sleepiness during the day. There was no association between daytime sleepiness category and grade level (Chi-squared = 18.2, df = 12, p = 0.10). The frequency of daytime sleepiness in boys was more common than in girls (Chi-squared = 30.8, df = 4, p < 0.001).
Falling asleep in school
About 32% of the students never experienced falling asleep in school and 28.9% experienced it less than once a week. Girls experienced this less than boys (Chi-squared = 21.4, df = 4, p < 0.001). The rate of falling asleep was not associated with grade level (Chi-squared = 13.1, df = 12, p = 0.35).
Intake of sleeping aids
About 95.8% of the study subjects never used medication to enhance sleep. The rate of medication use was not different between genders (Chi-squared = 6.7, df = 4, p = 0.15). In addition, it was not associated with grade level (Chi-squared = 9.1, df = 12, p = 0.68). About 54.3% of the students drank coffee one or more times per week in the evening and it was associated with onset of sleeping difficulties (Chi-squared = 38.7, df = 16, p < 0.001).
About 91% of the students never experienced bruxism and 1.4% of them experienced it more than four times per week (Figure 1). Bruxism was not related to either gender or grade level (Chi-squared = 3.1, df = 4, p < 0.5; Chi-squared = 9.7, df = 12, p < 0.6, respectively). The frequencies of nightmares, nocturnal eating habits, leg movement and other items are shown in Figure 2. Occurrence of nightmares was related to grade but not to gender (Chi-squared = 24.7, df = 12, p < 0.01; Chi-squared = 8.7, df = 4, p < 0.06, respectively). Gender and grade were not associated with nocturnal eating habits (Chi-squared = 9.3, df = 4, p < 0.5; Chi-squared = 18.7, df = 12, p < 0.09, respectively).