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                                                                                                549
550      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

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Sleep 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            1:01
Sleep 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 Society
554      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-

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Sleep 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.

                                                                           Hublin, C., Kaprio, J., Partinen, M. and Koskenvuo, M. Insufficient
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