There is a great difference between countries in duration of sleep time among students. The duration was 8 h 30 min for the Finish 1, more than 9 h for the Swiss 1, 6 h 20 min for the Japanese 2, 4.8 to 6.2 h for Koreans 3, 5.4 h in a multinational study 4, 7.5 h for the Chinese 5, and 7 to 8 h for American students 6. Additionally, sleep duration appears not to be gender-related 37.

A study on sleep/wake patterns among Korean teenagers showed that as the school year (grade) level increased, students went to bed later, woke up earlier, and slept less. Boys awoke significantly later than girls 3.

About 61% of the Korean students had not woken up at night during the previous 2 weeks, and the frequency increased across grades 9 to 12 from 51.3% to 56.3%, 63.0%, and 79.0%, respectively 3.

Sleep latency was longer in girls than boys 3. Daytime sleepiness was reported to be 19.9% 6, increasing with age in adolescents 1. Male adolescents fell asleep later than girls 4. About 14.9% of 12- to 18-year-old adolescents in China reported difficulty falling asleep on at least 4 nights in the past month 8.

A multinational study on adolescent sleep reported early morning waking in 10.5%, occurrence of nightmares in 4.2%, and sleepwalking in 5.0% 6. Breathing disorders (e.g. obstructive sleep apnoea) and primary snoring occurred in as many as 11% of the paediatric patients 9.

The prevalence of daytime sleepiness in teenagers significantly increased from grades 9 to 12 from 2.1% to 3.2%, 10.7%, and 13.9%, respectively and the prevalence was more in girls than boys. The rate of falling asleep three times or more per week in class was 18%. The rates significantly increased with grade at 4.0%, 5.5%, 11.6%, and 24.9%, respectively 3.

About 4% of Italian students had used medication to help themselves sleep at least once during the last 6 months and 1.3% had regularly taken sleeping pills. Girls were more prevalent users of sleep medication than boys 10.

Adolescents in Iran are from a generation whose parents have been involved in a long-lasting and devastating war and now the adolescents are directly and indirectly facing the many consequences of that war. In addition, there is no adequate time allocation for nap taking and eating breakfast or lunch at school. These factors do not seem to be present in the Western world.

This study surveys the self-reported night- and daytime sleep habits of adolescents in Shiraz, one of the largest cities in Iran. Most of the research in this field to date has been conducted in the Western world and, to our knowledge, there is no other study that has reported sleep patterns and sleep quality among Iranian adolescents, who are from a non-Western culture.

In order to take a sample that was representative of the group of subjects that we intended to study, eight all-male high schools and eight all-female high schools were randomly selected from strata defined by age, grade and sex. About 350 students (about 175 girls and 175 boys) were selected from each age group based upon stratified cluster sampling. A total of 1,500 students were selected, with 1,420 (94.6%) participating in the study. All subjects chosen were from Shiraz, one of the largest cities in Iran. All the students attending school on the day of the survey were invited to participate. The participating subjects were 1,420 high school students, of which 696 were boys (49.0%). The participants' mean age was 16.4 years and the age range was 15 to 18 years.

The questionnaire used was a comprehensive instrument including items about usual sleep/wake behaviours over the previous month. It asked about the schedule of sleep, falling asleep in class, difficulty falling asleep, tiredness or sleepiness during the day, and difficulty getting up in the morning. It also asked about occurrence of nightmares (dreams with content that the sleeper finds disturbing), leg movement (periodic episodes of repetitive and stereotyped limb movements in sleep), sleep talking (utterance of speech or sounds during sleep), sleep walking (a series of complex behaviours that result in movement during sleep), bruxism (grinding of the teeth during sleep), use of sleeping pills and whether the subject was in the habit of drinking coffee late in the evening 11. "Sleepiness during class" is different to "falling asleep in class". The first means that the subject is in a sleepy state, but is not actually asleep. The second one refers to actual sleeping. The instrument contained questions to which the subjects responded using a 1- to 5-point ordinal scale. It assessed the frequency of the respective problems during the week (1 = never, 5 = almost every day or night). Four questions required categorical answers and assessed what time the subject went to bed, the length of time needed to fall asleep, length of daytime naps, and the number of nocturnal awakenings. Sleep quality was rated from excellent (1) to very poor (5). Most questions concerned the whole week. The questionnaire was translated into Farsi and was back-translated to ensure that the questions retained the same meaning.

After a preliminary study to evaluate if the questions were understood as expected by the students, the final version was used in the current study. The questionnaire was administered by trained medical students in the period from 20 January to 20 February 2005 in high school classes during morning school hours. Participation of the students in the study was voluntary, and informed consent was taken from the participants, teachers and principals and the educational administrative headquarters in Shiraz. The study protocol was approved by the Ethics Committee of Shiraz University of Medical Sciences.

The data were analyzed using SPSS software for Windows (SPSS Inc., Chicago, IL, USA). The Student's t test and Chi-square tests were used to examine group differences for discrete data. Analysis of variance (ANOVA) was used to examine gender differences and grade effects on the variables. The level of significance was considered at p = 0.05.

The mean duration of night sleep was 7.7 h, being higher than that of Korean teenagers at 5.4 h 3. Also, it was more than Japanese adolescents 2. However, it was more similar to American adolescents at 7–8 h 46. Our result is contrary to those of previous studies that concluded sleep duration was shorter in Asia than in Europe 1213. There is a lot of emphasis on education during high school years in Iran, especially in order to be able to pass the national university entrance examination that is taken at the end of the high school period. Despite the expectation that students should shorten their sleep to study harder in all grade levels of high school, it seems that they usually sleep longer than in Korea and Japan.

Adolescents reported going to bed later as they got older. Studies from other countries such as Canada 14, Australia 15, and Brazil 16 reported similar findings. This might be related to less parental influence, more use of the computer, or on increases in the homework workload.

The mean waking time in the morning was 07.17 am and the time of waking in the morning was not different between genders and the mean was not different between grade levels. This is in contrast with findings from Korea, where boys woke significantly later than girls and the increase of grade level was associated with earlier waking 3. The Korean study assumed that these differences might result from differences in the time needed to prepare for school and/or family responsibilities. In Iran, female students wear a special uniform including a scarf and usually do not put on make-up. Therefore, preparation is faster and easier and they do not have to wake up earlier. Finally, the absence of significant gender difference in waking time might be a distinct characteristic of Iranian adolescents. It is important to note that school start time is not different for boys and girls and there are no co-educational schools (mixed sex) for adolescent boys and girls in Iran.

Sleep latency is common and the frequency of sleep latency categories was not associated with gender or grade level. This is not in accordance with the results of some previous studies 317. Another previous study reported that difficulty in falling asleep is the most common complaint among adolescents, and there is a trend indicating that longer sleep latency is more frequent in girls than boys 18.

About 44.6% of the students had not woken up at night during the previous 4 weeks, and the frequency increased with grade (grade 9, 10, 11, 12, from 40% to 40.2%, 51.1%, and 47%, respectively). These findings are very similar to the results of the Korean study 3.