Sleep patterns and insomnia among adolescents: a population-based study
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J Sleep Res. (2013) 22, 549–556 Adolescents and insomnia
Sleep patterns and insomnia among adolescents:
a population-based study
MARI HYSING1,2, STÅLE PALLESEN3,4, KJELL M. STORMARK1,6,
A S T R I J . L U N D E R V O L D 1 , 2 , 7 and B Ø R G E S I V E R T S E N 5 , 6
1
The Regional Centre for Child and Youth Mental Health and Child Welfare, Uni Health, Uni Research, Bergen, Norway, 2Department of
Biological and Medical Psychology, University of Bergen, Bergen, Norway, 3Department of Psychosocial Science, University of Bergen, Bergen,
Norway, 4Norwegian Competence Center for Sleep Disorders, Haukeland University Hospital, Bergen, Norway, 5Division of Mental Health,
Norwegian Institute of Public Health, Bergen, Norway, 6Department of Clinical Psychology, University of Bergen, Bergen, Norway and 7K. G.
Jebsen Centre for Research on Neuropsychiatric Disorders, University of Bergen, Bergen, Norway
Keywords SUMMARY
adolescents, epidemiology, insomnia, The aim of the current study was to examine sleep patterns and rates of
prevalence, sleep
insomnia in a population-based study of adolescents aged 16–19 years.
Correspondence Gender differences in sleep patterns and insomnia, as well as a
Dr Mari Hysing, Centre for Child and Adolescent comparison of insomnia rates according to DSM-IV, DSM-V and
Mental Health and Welfare, Postbox 7810, 5020 quantitative criteria for insomnia, were explored. We used a large
Bergen, Norway.
population-based study in Hordaland county in Norway, conducted in
Tel.: +55-58-86-98;
fax: +47-55-58-98-78; 2012. The sample included 10 220 adolescents aged 16–18 years (54%
e-mail: mari.hysing@uni.no girls). Self-reported sleep measurements included bedtime, rise time,
time in bed, sleep duration, sleep efficiency, sleep onset latency, wake
Accepted in revised form 8 March 2013;
after sleep onset, rate and frequency and duration of difficulties initiating
received 18 January 2013
and maintaining sleep and rate and frequency of tiredness and
DOI: 10.1111/jsr.12055 sleepiness. The adolescents reported short sleep duration on weekdays
(mean 6:25 hours), resulting in a sleep deficiency of about 2 h. A
majority of the adolescents (65%) reported sleep onset latency exceed-
ing 30 min. Girls reported longer sleep onset latency and a higher rate of
insomnia than boys, while boys reported later bedtimes and a larger
weekday–weekend discrepancy on several sleep parameters. Insomnia
prevalence rates ranged from a total prevalence of 23.8 (DSM-IV
criteria), 18.5 (DSM-V criteria) and 13.6% (quantitative criteria for
insomnia). We conclude that short sleep duration, long sleep onset
latency and insomnia were prevalent in adolescents. This warrants
attention as a public health concern in this age group.
of mental health problems (Cousins et al., 2011) as well as
INTRODUCTION
increased risk of traffic accidents (Danner and Phillips, 2008).
During adolescence a range of biological, psychological and Insufficient sleep has been defined as the duration of sleep
social factors interact, resulting in shortened sleep duration, below which waking deficits begin to accumulate (van
in what has been characterized as ‘the perfect storm’ Dongen et al., 2003), whereas sleep need has been defined
(Carskadon, 2011). Secular trends suggest that sleep defi- as habitual sleep duration in the absence of pre-existing
ciency and sleep problems are increasing among adoles- sleep debt (Dement and Grenber, 1966). Most studies rely
cents (Matricciani et al., 2012; Pallesen et al., 2008). As this solely upon sleep duration as an indicator of insufficient
shortened sleep duration is not accompanied by a reduction sleep, which may be inaccurate due to large variations in
in sleep need during adolescence, a large proportion of individual sleep need (Mercer et al., 1998). Others have
adolescents experience sleep deficiency or insufficient sleep defined insufficient sleep as sleep duration far below age-
(Fallone et al., 2002), with possible negative consequences expected norms (Pallesen et al., 2011) or ask explicitly
in terms of reduced daytime functioning and school perfor- whether and to what degree the respondent has not obtained
mance (for a review, see Dewald et al., 2010), increased risk enough sleep (Altman et al., 2012). Perhaps a better
ª 2013 European Sleep Research Society 549550 M. Hysing et al.
approach to making it possible to express the magnitude of adolescence, and potential gender differences based on
insufficient sleep is to calculate the discrepancy between DSM-V and research criteria for insomnia in adolescence
self-reported sleep and perceived sleep need. This approach have, to the best of our knowledge, not been investigated.
has also been used in previous studies (Hublin et al., Based on the above considerations, the aim of the present
2001). study was to characterize sleep patterns in adolescence,
Adolescent sleep is characterized by a large discrepancy including gender differences, in a large population-based
between weekdays and weekend sleep patterns, including a study. The second aim was to assess the rate of insomnia
sleep phase shift to later bedtimes as well as an average of 1 according to the definitions of DSM-IV, DSM-V and quanti-
and 2 h longer sleep durations during weekends (Crowley tative criteria for insomnia, analysed separately for girls and
et al., 2007; Gradisar et al., 2011a). While adolescent sleep boys.
phase delay is well documented (Taylor et al., 2005), less is
known about the time it takes adolescents to fall asleep, as
METHODS
assessment of sleep onset latency (SOL) is seldom included
in general population-based studies. In a recent review of In this population-based study, we employed information
studies on sleep patterns among adolescents, only three from the ung@hordaland survey of adolescents in the county
single studies assessed SOL. These showed that between of Hordaland in western Norway. All adolescents born
20 and 26% of the samples took more than 30 min to fall between 1993 and 1995 and all students attending second-
asleep (Gradisar et al., 2011b). An Icelandic study reported ary education during spring 2012 were invited to participate in
an average SOL of 16.8 min among adolescents (Thorleifs- the ung@hordaland survey, the main aim of which was to
dottir et al., 2002). The authors proposed that the definition of assess mental health problems and service use in adoles-
an acceptable/normal SOL may differ between adults and cents, with a special emphasis on the prevalence of mental
adolescents. This issue has not been resolved, and inclusion health problems. The data were collected during spring 2012.
of SOL as a central sleep parameter in future studies of sleep Adolescents in upper secondary education received informa-
among adolescents is therefore recommended (Gradisar tion via e-mail, and one school hour was allocated for them to
et al., 2011b). complete the questionnaire at school. Those not at school
Prolonged SOL is also a defining characteristic of insomnia. received information by postal mail to their home addresses.
The exact rate of insomnia among adolescents is uncertain The questionnaire was web-based, and covered a broad
(Roane and Taylor, 2008) due to large variations in opera- range of mental health issues, daily life functioning, use of
tionalization across studies, thus complicating comparisons. health care and social services, demographic background
In a European general population study of adolescents aged variables and a request for permission to obtain school data,
between 15 and 18 years, a 4% prevalence rate of insomnia and to link the information with national health registries and
according to the DSM-IV criteria was found (Ohayon et al., parental questionnaires. Uni Health collaborated with Horda-
2000). An American population-based study reported a land County Council in conducting of the study. The study
10.7% lifetime prevalence of insomnia according to the was approved by the Regional Committee for Medical and
DSM-IV criteria, including a frequency criterion of 4 days a Health Research Ethics in western Norway. The current
week (Johnson et al., 2006). The proposed revision in the study is based on the first version of data files released in
DSM-V concerning insomnia disorder will probably affect the May 2012.
estimated prevalence, as it adds a minimum frequency
criterion of 3 days to the diagnosis and increases the minimal
Sample
duration threshold from 1 to 3 months. Similarly, estimated
prevalence rates are different when using the suggested All adolescents born between 1993 and 1995 (n = 19 430)
quantitative criteria for insomnia, recommending an opera- were invited to participate in the current study, which took
tionalization of insomnia based on a thorough review of the place during the first months of 2012, 10 220 of whom
literature in order to identify the most valid criteria (Lichstein agreed, yielding a participation rate of 53%. Sleep variables
et al., 2003). These criteria specify a SOL of more than were checked for validity of answers based on preliminary
30 min, and insomnia occurring on three or more nights a data analysis, resulting in 374 subjects being omitted due to
week for at least 6 months. The prevalence of insomnia using obvious invalid responses (e.g. negative sleep duration and
the new proposed DSM-V definition or the quantitative criteria sleep efficiency). Thus, the total sample size in the current
has, to our knowledge, not been assessed previously in study was 9875.
population samples of adolescents.
While no gender differences in prepubertal children in
Instruments
insomnia rates were found, a more than twofold risk for
insomnia was found in postmenes girls compared to boys in
Demographic information
an American population-based study using the DSM-IV
diagnostic criteria (Johnson et al., 2006). However, there All participants indicated their vocational status, with
are few studies regarding gender differences in insomnia in response options being ‘high school student’, ‘vocational
ª 2013 European Sleep Research SocietySleep in adolescence 551
training’ or ‘not in school’. Maternal and paternal education and (ii) sleepiness and/or tiredness (see above). In addition,
were reported separately, with four response options: to fulfil the criteria for DSM-IV insomnia, duration of DIMS for
‘primary school’, ‘secondary school’, ‘college or university: at least 1 month was also required. For DSM-V insomnia, the
less than 4 years’ and ‘college or university: 4 years or following additional criteria were required: a DIMS frequency
more’. of at least 3 days per week and duration of insomnia of at
least 3 months. The quantitative criteria for insomnia (Lich-
stein et al., 2003) were operationalized as follows: reporting
Sleep variables
DIMS at least three times a week, with a duration of 6 months
Self-reported bedtime and rise time were reported sepa- or more, in addition to reporting SOL and/or WASO of more
rately for weekends and weekdays. Time in bed (TIB) was than 30 min.
calculated by subtracting bedtime from rise time. SOL and
WASO were reported in hours and minutes, and sleep
Statistics
duration was defined as TIB minus SOL and WASO.
For the purpose of the present study, sleep duration was IBM SPSS version 20 (SPSS Inc., Chicago, IL USA) for
also split into ten categories (12 h). Sleep efficiency was variance (MANOVA) was used to examine gender and age
calculated as sleep duration divided by TIB multiplied by differences on the sleep variables, as well as interaction
100 (reported as a percentage). Subjective sleep need effects between age and gender. Chi-square tests were
was reported in hours and minutes, and sleep defi- used to examine differences in sleep duration (10 different
ciency was calculated separately for weekends and week- duration categories) between weekdays and weekends.
days, subtracting total sleep duration from subjective sleep Gender differences in insomnia prevalence were estimated
need. using logistic regression analyses using gender as the
Difficulties initiating and maintaining sleep (DIMS) were exposure variables and the three insomnia definitions (DSM-
rated on a three-point Likert scale, with response options of IV insomnia, DSM-V insomnia and quantitative criteria for
‘not true’, ‘somewhat true’ and ‘certainly true’. Given a insomnia) as outcome variables. To investigate whether
positive response (‘somewhat true’ or ‘certainly true’), the gender differences were significantly different across the
participants were then asked how many days per week they three operationalizations, we also calculated the relative risk
experienced problems either initiating or maintaining sleep. ratio (RRR), as recommended by Altman and Bland (2003),
The participants also provided information on the duration of in order to test for significant differences between the odds
DIMS. ratios. This is a well-established test of interaction to
A joint question on tiredness/sleepiness was rated on a compare estimates on a log scale.
three-point Likert-scale with response options of ‘not true’,
‘somewhat true’ and ‘certainly true’. If confirmed (‘somewhat
true’ or ‘certainly true’), participants reported the number of
days per week on which they experienced sleepiness and Table 2 Demographical variables in the ung@hordaland study
tiredness, respectively. (n = 9846)
Three operationalizations of insomnia were investigated
Mean SD
(see Table 1). All three definitions included a positive
response (‘somewhat true’ or ‘certainly true’) to (i) DIMS Age (years) 17.0 0.87
% n
Girls 53.5 5215
Table 1 Overview of the insomnia diagnosis in DSM-IV, the
Vocational situation
proposed revision in DSM-V and the quantitative criteria for
High school student 97.9 9219
insomnia
Vocational training 1.4 132
Duration Frequency Not in school 0.7 67
of of SOL and Daytime Maternal education
insomnia insomnia WASO functioning Primary school 7.7 742
Secondary school 31.4 3042
Diagnostic criteria for insomnia College/university (6 months 3 nights/ ! 31 min X College/university (552 M. Hysing et al.
Fig. 2 for details). SOL was significantly longer for girls than
RESULTS
boys (P < 0.001). Mean WASO was 15 min, and 79% of the
Of the adolescents born between 1993 and 1995 (with a adolescents reported less than 15 min WASO. Mean sleep
mean age of 17 years), 53.5% of the participants were girls; efficiency during weekdays was 85%, with girls (84%) having
the majority comprised high school students (98%). For lower sleep efficiency than boys (87%). Higher sleep
details and information on socioeconomic status and demo- efficiency was observed at the weekend, but with similar
graphic information, see Table 2. gender differences (88% versus 90% for girls and boys,
respectively (P < 0.001).
Sleep patterns
Insomnia
Bedtime, rise time, TIB and sleep duration for the total
sample, stratified by gender, are presented in Table 3. The The prevalence of insomnia was calculated for the total
mean bedtime on weekdays was 23:18 hours, significantly sample and separately for the two gender groups using three
later for boys (23:56) than girls (23:10). Mean TIB for different definitions. All insomnia definitions included an algo-
weekdays was 7:29 hours (boys: 7:26 and girls: 7:32), rithm of difficulties initiating and/or maintaining sleep and
whereas mean sleep duration was 6:25 hours (boys: 6:28 tiredness and/or sleepiness during daytime. The prevalence
and girls: 6:22). estimates ranged from a total prevalence of 23.8%, using the
Bedtime during weekends was, on average, 2 h and DSM-IV criteria, to 18.5% according to the proposed DSM-V
25 min later than on weekdays (01:13), while the corre- criteria, expanding the duration from 1 to 3 months and
sponding rise time discrepancy was 4 h and 28 min, reflect- including frequency criteria of 3 days per week (See Fig. 3).
ing that the adolescents slept on average 2 h and 12 min According to the quantitative criteria for insomnia, the
more during weekends than on weekdays. Both rise- and prevalence was 13.6%. Girls had a significantly higher
bedtime discrepancies between weekdays and weekends prevalence of insomnia across all three insomnia definitions,
were significantly larger for boys than girls (P < 0.001). but there were no significant differences in the magnitude of
Distribution of sleep duration on weekdays and at weekends gender differences between the three diagnostic criteria, as
are presented in Fig. 1. calculated by the RRR (all 95% confidence intervals included
The adolescents’ subjective sleep need was 8 h and the value 1.0).
35 min, yielding a sleep deficiency on weekdays of 2 h and
9 min. No sleep deficiency was found for weekends.
Age, gender and interaction effects
Significant gender differences were found for most sleep
SOL, WASO and sleep efficiency
variables, as detailed in Table 3. There was a significant age
Mean SOL was 47 min, with 24.2% reporting SOL less than effect on some sleep variables, with the youngest age
15 min and 59% reporting SOL longer than 30 min (see cohorts reporting earlier bedtimes and rise times both on
Table 3 Sleep characteristics in the ung@hordaland study (n = 9846)
Girls Boys Total
Mean SD Mean SD P-value Mean SD
Weekdays
Bedtime 23:10 0:57 23:26 1:01Sleep in adolescence 553
Figure 1. Sleep duration on weekdays and at weekends. Error bars represent 95% confidence intervals.
Figure 2. Sleep onset latency (SOL) and wake after sleep onset (WASO) among adolescents in the ung@hordaland study (n = 9846).
weekdays and at weekends, and also sleeping longer at bedtimes, long SOL and a short sleep duration, contributing
weekends. The youngest cohort also spent more TIB, had to a daily sleep deficiency of about 2 h on weekdays. A high
larger subjective sleep need, had more sleep deficiency rate of insomnia was evident across the diagnostic defini-
during weekdays and reported less insomnia than older tions, with total prevalence ranging from 23.8% using the
adolescents (all Ps < 0.001). There were no significant age DSM-IV criteria to 18.5% according to the proposed DSM-V
differences in SOL, WASO and sleep efficiency. criteria and to 13.6% using the quantitative criteria for
Few significant interaction effects between age and gender insomnia. Girls had a significantly higher prevalence of
were observed, except for bedtime, rise time and TIB during insomnia than boys across all three insomnia definitions.
weekdays (all Ps < 0.001). For example, while TIB increased Information about sleep patterns during weekdays revealed
with age for girls, a corresponding decrease with age was late bedtimes. Rise time on school days showed limited
observed for boys (P < 0.001). variations, due probably to the fact that most of the
adolescents were high school students with fixed and early
school starting times. To obtain the recommended sleep
DISCUSSION
duration of 8–9 h, which was also in accordance with their
To sum up the main findings: sleep patterns of adolescents self-perceived sleep need, they should have gone to bed at
between 16 and 19 years were characterized by late around 22:00 hours. At weekends, their sleep duration was in
ª 2013 European Sleep Research Society554 M. Hysing et al.
Figure 3. Prevalence of insomnia according to different operationalizations, stratified by gender. Error bars represent 95% confidence intervals.
accordance with their subjective sleep need, with a shift adolescents reporting sleep onset difficulties (Pallesen
towards later bedtime of more than 2 h and an even later rise et al., 2008).
time. The adolescent’s sleep pattern in the present study Another possible reason for the observed discrepancies
confirmed sleep phase delays and the late bedtimes reported between the findings in the current and previous studies may
in previous studies of adolescents (Crowley et al., 2007; be seasonal variations in terms of daylight illumination during
Gradisar et al., 2011b). the time of data collection. The data in the current study were
The mean sleep duration of approximately 6½ h on collected from February to May 2012, and while we cannot
weekdays shown in the present study is, however, shorter rule out a possible seasonal influence on our data, a recent
than that reported in most previous studies. One reason study from Norway investigating seasonal variation in insom-
could be differences in definitions, methods and samples nia and sleep duration found no evidence of such a seasonal
between our and other studies of adolescent sleep duration. effect (Sivertsen et al., 2011).
For example, in the Ohayon study the mean sleep duration The adolescents reported a mean sleep need of between 8
in a European sample of 15–18-year-olds was approxi- and 9 h. While there are individual differences, the mean
mately 8 h (Ohayon et al., 2000). This was methodologically reported sleep need in the present study is in accordance
comparable to the present study, as both calculated sleep with the empirically derived suggested sleep need in
duration by subtracting sleep latency from bedtime. Other adolescents of about 8–9 h (Carskadon et al., 1980). In the
studies have used more dissimilar definitions. We have, for present study, their sleep duration is in accord with their
example, defined sleep duration by subtracting reported subjective sleep need during weekends, as they probably do
wake time (SOL and WASO), whereas results from many not have such specific demands or obligations in the
previous studies are based on TIB only. In their review of morning. This underscores that the sleep duration during
international sleep studies, Gardiner et al. suggested that weekdays are too short. As short sleep duration is known to
the reported sleep duration was probably overestimated by be related to a range of impairments in terms of academic
about a quarter of an hour. The review of international sleep functioning (Dewald et al., 2010), overweight (Danielsen
studies (Gradisar et al., 2011b) suggested that the sleep et al., 2010), depressive symptoms and mental health
duration reported in their overview was probably over- problems in general (Cousins et al., 2011), there are reasons
estimated, as it was calculated based on the SOL in an to be concerned about the sleep habits of the majority of
icelandic study that reported SOL of about 15 minutes adolescents included in the present study.
(Thorleifsdottir, et al., 2002). Mean SOL was 47 min in the There was a high rate of insomnia in the present study,
present study, which is more than twice as much as ranging from 13.6% to 23.6%, depending on diagnostic
reported previously in studies of SOL in adolescents criteria. The paucity of studies assessing insomnia in
(Gradisar et al., 2011b), and a considerably longer mean adolescents and the different definitions used hinders com-
SOL than reported in the aforementioned Icelandic study parisons across studies, but the prevalence is higher than the
(Thorleifsdottir et al., 2002). The fact that the SOL was estimated rates of between 4 and 10% reported in previous
longer in the present study than in most previous studies studies using DSM-IV insomnia criteria (Johnson et al., 2006;
may reflect cohort effects. This interpretation is in line with Ohayon et al., 2000). Another potential limitation in the DSM-IV
studies showing a secular increase in the proportion of definitions concerns the independence of co-occurring psychi-
ª 2013 European Sleep Research SocietySleep in adolescence 555 atric disorders. While the DSM-IV differentiates between in girls (Johnson et al., 2006). While the results support the primary insomnia and insomnia related to another disorder, importance of gender-specific analysis in this age group, the we have not differentiated between these subtypes in order to mechanisms leading to these differences are, by and large, ease comparison with the other diagnostic systems. Another unknown. While the shorter TIB and shorter sleep duration limitation of the present study relates to the exclusive use of found in boys could be due to the lower perceived sleep questionnaire-based information. Thus, diagnostic interview need that they reported in the present study, the differences data, the gold standard for diagnosing insomnia, was not could also be related to later pubertal development in boys, collected, but we relied instead upon a broad range of as there was an interaction effect of gender and age on questionnaire-based sleep parameters that were used in some sleep patterns; e.g. bedtime and rise time during accordance with specific diagnostic definitions. The criteria weekdays. However, there was no interaction effect for daytime functional impairment in the present study were between age and gender on insomnia and sleep efficiency, tiredness and sleepiness assessed by a joint variable. and thus pubertal developmental levels are less likely to Although sleepiness is used more commonly as a symptom account for these differences. As insomnia and lying awake of obstructive sleep apnea, we chose to include both tiredness in bed has been found to be related to worrying and and sleepiness in the operationalization of insomnia due to a depressive symptoms, some of the gender differences in large overlap of these terms in the Norwegian language and sleep efficacy, SOL and WASO may be related to the due to limited ability in lay-people to discriminate between the parallel increased rate of depression in girls after puberty two constructs. (Danielsson et al., 2012). A final limitation relates to how sleep duration was The strengths of the present study include the combination calculated. No data on time awake in bed prior to putting of large sample size and inclusion of a broad range of sleep the light out and lying awake prior to rising were collected, parameters. The attrition from the study could affect gener- and thus TIB – (SOL + WASO) might also include such alizability, with a response rate of approximately 53% and periods. This may, potentially, lead to sleep duration being with adolescents in schools over-represented. Based on even shorter than reported in the current study. previous research from the former waves of the Bergen Child The present study has shown that prevalence rates are Study, non-participants often have more psychological prob- influenced strongly by the quantitative operationalization and lems than participants (Stormark et al., 2008). definitions of insomnia. While including a cutoff for SOL and a How can we help adolescents to achieve a sleep duration longer duration of the symptoms based on the quantitative in agreement with their self-perceived need, and also their criteria for insomnia, nevertheless a high percentage (13.6%) need according to recommended guidelines? The magni- was diagnosed with insomnia. While this could mirror the high tude of the problem demonstrated in the present study rate of true insomnia in adolescents, it could also reflect the indicates that short sleep duration is a public health issue. need for adjustment of some of the criteria related to the The results from previous school-based sleep education respondents’ age/maturation, for instance. The cutoff intervention studies for adolescents show positive effects on (>30 min) for SOL used in the research criteria is based on knowledge, but not on basic sleep habits (Cain et al., 2011; epidemiological studies of adults. In adolescence, a long Moseley and Gradisar, 2009). In addition, the adolescents in SOL seems to be the norm and may reflect pubertal-related the present study seem to be aware of the discrepancy delayed circadian rhythms (Taylor et al., 2005) as well as between their obtained sleep during weekdays and their pubertal slowing of the homeostatic sleep drive build-up sleep need. In an Australian study, parent-set betimes were (Jenni et al., 2005). Thus, the appropriate cutoff for SOL in found to be related to longer sleep duration and improved determining insomnia in adolescents has still not been daytime functioning, suggesting that parents may be key in settled. Nocturnal awakenings seems to be less frequent in improving adolescents sleep (Short et al., 2011). However, adolescents, thus the present criterion (>30 min) might be as this was an observational study, further intervention acceptable. As the present study indicates, the duration studies are needed. Few older adolescents will probably criteria will have only minor effects on the prevalence rates as regard such an approach as acceptable, as fewer than 10% most of the adolescents report insomnia for more than of high school students seem to have their bedtimes set by 1 year. Based on the present study, we cannot decide what parents (Carskadon and Acebo, 2002). On a societal level, is the most accurate or optimal operationalization. How these later school starting times have been suggested as a means definitions are related to outcome measures in terms of of improving adolescents sleep (Carrell et al., 2011; Danner functioning/impairment, other co-occurring disorders and the and Phillips, 2008; Kirby et al., 2011; Vedaa et al., 2012). associated level of distress should, in future, be used as More research is needed to both pinpoint the mechanisms guidelines for choosing the most useful definitions. leading to short sleep, including the effect of electronic media Gender differences emerged regarding sleep patterns and (Cain and Gradisar, 2010). The current findings emphasize insomnia prevalences irrespective of insomnia definition, that sleep problems among adolescents are a significant showing a considerable female preponderance. This is in public health concern, and that low-threshold interventions accord with findings in some previous studies, especially the and prevention programmes should be targeted for this age gender-typical pattern of higher insomnia rates after puberty group. ª 2013 European Sleep Research Society
556 M. Hysing et al.
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