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Sleep. Jul 1, 2011; 34(7): 885–890.
Published online Jul 1, 2011. doi:  10.5665/SLEEP.1120
PMCID: PMC3119830

Sleep Duration and Overweight in European Children: Is the Association Modified by Geographic Region?

Sabrina Hense, MA, MSc,1 Hermann Pohlabeln, PhD,2 Stefaan De Henauw, MD, MSc, PhD,3 Gabriele Eiben, PhD,4 Dénes Molnar, MD,5 Luis A. Moreno, PhD,6 Gianvincenzo Barba, MD,7 Charalampos Hadjigeorgiou, MD,8 Toomas Veidebaum, MD,9 and Wolfgang Ahrens, PhD1, on behalf of the IDEFICS Consortium


Study Objectives:

To investigate differences and a possible effect modification by geographical region in the association between sleep duration and overweight.




Primary schools and preschools in 8 European countries.


7867 children aged 2 to 9 years.


Not applicable.


Nocturnal sleep duration was assessed as part of a parental 24-h recall. Height and weight were measured by standardized procedures across centers. Data on personal, social, environmental and behavioral factors were collected using a standardized parental questionnaire.


Sleep duration differed (P < 0.001) between European regions and normal vs. overweight children. A dose-dependent inverse association between sleep duration and overweight could be seen, with crude odds ratios ranging from 1.73 (99% CI 1.33; 2.25) for sleeping between 10 and 11 h to 3.81 (99% CI 2.85; 5.09) for sleeping less than 9 h (reference category > 11 h). This persisted after adjustment, but remained significant only for sleeping less than 9 h per night (north: OR = 1.70; 99% CI 1.13; 2.58 vs. south: OR = 2.84; 99% CI 1.57; 5.12) if stratified by region. No effect modification by region could be found, but adjustment for region accounted for changes in the effect estimate for sleeping less than 9 h (OR = 2.22; 99% CI 1.64; 3.02). The association was stronger in school children than in preschool children.


Geographic region and related aspects—even if they do not seem to modify the association between sleep and overweight—should in any case be taken in consideration as a confounding factor on this association.


Hense S; Pohlabeln H; De Henauw S; Eiben G; Molnar D; Moreno LA; Barba G; Hadjigeorgiou C; Veidebaum T; Ahrens W. Sleep duration and overweight in European children: is the association modified by geographic region? SLEEP 2011;34(7):885-890.

Keywords: Epidemiological study, school children, preschool children, cross-sectional, IDEFICS


An adequate amount of sleep is believed to be important for optimal health and functioning throughout life, and the effect of sleep duration on diverse health outcomes seems undisputable.1 Concurrent with the worldwide increase of overweight and obesity, a decrease in sleep duration has been observed and mounting evidence in the recent years has identified sleep duration as a potentially new risk factor for overweight and obesity already in childhood.18 Also an association between short sleep duration and metabolic dysfunction has been discussed.911 Several studies suggest an influence of sleep on hormones including leptin, ghrelin, insulin, cortisol, and growth hormones. These hormonal changes potentially result in hormonal imbalances accounting for overweight and obesity.12 Since overweight children often become overweight or obese adults, which increases the risk for secondary diseases including type 2 diabetes or cardiovascular diseases,2,3,5,13 this risk factor may have a relevant impact on public health and needs to be further examined with special focus on children.

Data on sleep duration and its relation to overweight have been published for different age groups from several countries1,12,1418 but comparability of those results is limited by reasons of different study designs and methods. Moreover, it can be assumed that sleep duration as well as body mass index (BMI) are likely to be influenced by several cultural and environmental factors.15,1921 The present study reports data from an epidemiological survey conducted in children from 8 European countries. Data were collected within the framework of an international collaboration, according to a standardized protocol2224 and therefore allow for comparability and internal consistency of data. The study aims to investigate if there are significant differences in the association between sleep duration and overweight in diverse culture areas such as northern and southern Europe, where children are exposed to a broad range of different sleep durations25 and if regional affiliations may even act as an effect modifier for this association. The possibility to statistically test for regional differences in the association is only possible, if a large data set and internationally comparable data is available. The present study gives an opportunity to appropriately investigate on these regional effects. Moreover the high degree of standardization of data collection in a multicenter study allows for comparability of the data, while the comparison of different studies is hampered by methodological heterogeneity. Thus the present study leaves less room for the occurrence of differences that can be attributed to methodological differences than a comparison of different studies from different countries would do.


The “identification and prevention of dietary- and lifestyle-induced health effects in children and infants” (IDEFICS) project is a population-based multicenter study which includes children aged 2 to 9 years from 8 European countries. Between September 2007 and May 2008, 31,543 children from schools and preschools in selected regions in Italy, Estonia, Cyprus, Belgium, Sweden, Hungary, Germany and Spain were asked to participate in the baseline survey (T0), and 16,864 (53.4%) accepted the invitation. Of those children 16,223 (51.4%) gave full information on sex, height, and weight and thus fulfilled the inclusion criteria. Data on BMI and sleep duration were available for 10,613 (65.4%) of 16,223 children. After exclusion of children reporting only sleep duration for weekend days and those without full information on all covariates, 7,867 (48.5%) children were eligible for the present analysis (more information on sleep data and the covariates is given in the respective paragraphs). Children with information on sleep duration did not differ from children without sleep information in terms of sex, while there were some significant differences concerning age in southern Europe, where children with sleep information were older than those without sleep data (mean difference 0.2 years, P < 0.001). No significant differences in age were seen in northern countries.

Two age groups were created with one group including children aged 2 to < 6 years (preschool children) and the other group including children aged 6 to 9 years (school children). Additionally, age was included in the analysis as a continuous variable with increments of one decimal. Since previous studies,19,26 as well as our own data25 suggest that sleep habits and bedtime routines differ between southern and northern European countries, we decided to create two regional groups with southern Europe including Italy, Spain, Hungary and Cyprus and northern Europe including Sweden, Belgium, Estonia and Germany. In each country, the participating centers obtained ethical approval by their responsible authority. All children and their parents provided oral and written informed consent respectively for all examinations and/or the collection of samples, subsequent analysis and storage of personal data and collected samples. More detailed information on the study procedures are published elsewhere.22,24


Anthropometric measurements were done according to a standardized manual in all centers. Body height was measured without shoes by trained staff using a portable stadiometer (SECA 225). Weight was measured by means of an adapted version of electronic scale (TANITA BC 420 SMA), with subjects wearing only underwear. Body mass index (BMI) was calculated and then categorized referring to cutoff points according to the criteria of International Obesity Task Force.27 Our category “overweight” included overweight and obese children, while the reference category included normal and underweight children.

Sleep Duration

Information on sleep duration was collected in the context of a standardized 24-h dietary recall (SACINA). SACINA is a computer-based instrument filled out by parents or guardian of each participating child and contained questions on the time at which the child got up in the morning and went to bed on the previous day. Data were collected on all days of the week, including weekends. However, weekday sleep is likely to be more representative of usual sleep duration, because children in the relevant age group are expected to have a more regular bedtime and get-up routine on weekdays than during the weekend. In fact, analysis of variance confirmed no significant differences in sleep duration from Monday to Thursday while there were significant differences between Fridays, Saturdays, and Sundays (P < 0.001). Therefore, only sleep duration data for the nights from Monday to Thursday were included in the analysis. If data on sleep duration was given for more than one night, the first weekday night was included in the analysis to ensure homogeneity of the variability within the data.

Nocturnal sleep duration was calculated as the difference between bed- and get-up time in the SACINA interview resulting in a continuous variable. For our multivariate analysis we created 4 sleep categories (≤ 9 h, > 9 h to ≤ 10 h, > 10 h to ≤ 11 h, > 11 h). The smallest number of subjects per category was 1,441 (sleeping more than 11 h).

Confounding Variables

Data on personal, social, environmental, and behavioral factors of each child, such as time spent in front of a television or computer screen (screen time) and parental education level was collected by means of a standardized parental self-completion questionnaire. Education level was categorized according to the International Standard Classification of Education (ISCED). Two levels of education (low vs. medium/high) were created out of the 6 ISCED levels, with ISCED levels 0-2 being defined as low education, and ≥ 3 being defined as medium or high education. Information on dietary habits was obtained from a standardized Food Frequency Questionnaire. Since recent research suggests an association between fat consumption and sleep duration,28 a fatty food variable was included in the present analysis. Consumption of fatty foods was calculated by summing consumption frequencies of fried potatoes, whole fat milk, whole fat yoghurt, fried fish, fried meat, butter/margarine, savory pastries and fritters and fried eggs and dividing it by consumption frequency of all food items. The derived propensity scores depicted the percentage of fatty food consumption resulting in a continuous variable.

Children's physical activity was surveyed with accelerometers (Actigraph GT1M and ActiTrainer) with cutoffs for moderate to vigorous physical activity (MVPA) according to Sirard et al.,29 resulting in a continuous variable of daily minutes spent in MVPA. The accelerometers were worn around the waist for 3 consecutive days and had to be taken off for swimming, showering or bathing, and during sleep. The parents were asked to keep records of each time the device was taken off in an accelerometer diary. Information on daylight duration was obtained by month for each study center from astronomical tables available at www.timeanddate.com. Daylight duration is given in decimal hours. Temperature data (monthly average) was based on climate tables at www.woeurope.eu. Both, daylight duration and temperature were included as continuous variables in the analysis.

Statistical Analysis

Differences in continuous variables were compared using the Student t-statistics, while the χ2 statistic was used for comparison of categorical variables. To estimate the association between sleep duration and overweight controlled for confounding factors, we fitted logistic regression models, including variables that were related with both, sleep and overweight in univariate analysis and that accounted for ≥ 10% change in the estimate for sleep duration in our general model (model 1). The significance level was set at α = 0.01 to account for the elevated sample size. In a second step we conducted the same regression stratified by European region (northern vs. southern) to check for possible differences in the effect estimate for sleep duration categories (model 2). Subsequently we created our final model including region as an additional control variable (model 3). Tests for interaction were conducted, including interaction terms for region per sleep duration and age group per sleep duration in the final model. Statistical analysis was done with SAS Version 8.2 (SAS Institute, Cary, North Carolina, USA).


There were slightly less children from southern than from northern Europe in the present study sample and the proportion of school children was a little higher than the proportion of preschool children. The proportion of boys and girls was about equal (Table 1).

Table 1
Characteristics of study sample and associations of possible confounder variables with overweight and minutes of sleep duration

Student t-statistics indicated that school children slept on average 0.33 h (99% CI 0.28; 0.39) less than preschool children. Sleep duration also differed significantly between overweight and normal weight children, with normal weight children sleeping on average 0.34 h (99% CI 0.28; 0.41) more than overweight children. Accordingly, sleeping > 10 h (median of sleep distribution in present data) was more common among normal weight children than among overweight children (56.7 vs. 41.0%, P < 0.001). Figure 1 shows the distribution of sleep duration by weight status. A clear gradient in sleep duration could also be seen between regions, with children from northern Europe sleeping 0.59 h (99% CI 0.54; 0.64) longer than children in southern Europe. The percentage of children who slept > 10 h per night was 40.1% in southern and 65.5% in northern countries (P < 0.001). The distribution of sleep duration by region is shown in Figure 2.

Figure 1
Percentage of normal and overweight children by hours of sleep
Figure 2
Percentage of children from northern and southern Europe by hours of sleep

An overview of the association between possible confounder variables and sleep as well as overweight is given in Table 1. Age and temperature fulfilled the criteria for inclusion in multivariate analysis, and we therefore included them as covariables in our models.

A dose-dependent association between sleep duration and overweight could be observed in all models (Table 2). Taking a sleep duration ≥ 11 h as the reference category, the crude ORs for sleep duration ranged from 1.73 (99% CI 1.33; 2.25) for 10 to 11 h to 3.81 (99% 2.85; 5.09) for ≤ 9 h. In model 1 the associations persisted and were still significant if adjusted for the above mentioned co-variables with ORs of 1.36 (99% CI 1.05; 1.79) for 10 to 11 h and 2.89 (99% CI 2.15; 3.89) for ≤ 9 h of sleep. Similar results to those of model 1 could be seen in stratified analysis (model 2), even if not all associations remained significant. Estimates for sleeping ≤ 9 h differed slightly between northern and southern Europe, with the ORs being 1.70 (99% CI 1.13; 2.58) in the north and 2.84 (99% CI 1.57; 5.12) in the south. Also model 3 showed a dose-dependent relationship, with sleeping < 9 h accounting for a more than twofold risk of being overweight (OR = 2.22; 99% CI 1.64; 3.02). Inclusion of an interaction term did not show significant results (P = 0.075) and accounted for an only marginal change in estimate in this model.

Table 2
Associations of sleep duration and overweight (reference > 11 h)

Stratified by age, model 1 resulted in higher estimates for school children than for preschool children. In the analysis stratified by region (model 2), no association between sleep and overweight could be seen in preschool children. In school children the estimates were higher in northern countries than in southern countries, where the only significant association and the highest estimate was observed for sleeping < 9 h (OR = 3.20; 99% CI 1.43; 7.17). If adjusted for geographic region (model 3), sleeping 9 to 10 h (OR = 1.88; 99% CI 1.23; 2.86) and < 9 h (OR = 3.53; 99% CI 2.24; 5.54) was associated with overweight in school children, while no such association was observed in preschool children. The interaction term for region per sleep duration was not statistically significant nor did it account for relevant changes in estimate in any of the age groups.


Our multivariate models confirmed the observation made in bivariate analyses, that shorter sleep duration is associated with a higher probability of being overweight. The association though appeared to be much stronger in school children than in preschool children. Our data showed only weak evidence that regional differences might act as an effect modifier in the association between sleep duration and overweight. However, adjustment for geographic region attenuated the association between sleep duration and overweight to some degree.

Data on sleep duration of children has been presented in studies from different countries and for different age groups12,16,30,31 and attempts to define a “normal” sleep duration16 have been undertaken. Our study demonstrated that sleep duration differs between regions even within Europe. Children from southern Europe seemed to sleep less than children from northern countries. In fact, cultural differences in sleep duration have been reported also in other studies.15,26 The advantage of our study is that it presents comparable epidemiological data for European regions. As for the association between sleep duration and obesity, our data confirm findings from other cross-sectional studies.15,7,8 Even if previous findings were largely consistent between countries and continents with regard to the direction of the association,7 differences in effect sizes have been observed.3,12 However, comparison of those results is hampered, due to differing study designs and methods. To account for this limitation we wanted to find out, whether the association between sleep duration and overweight would differ between geographic regions, in this case between children from northern and southern European countries. Differences between effect estimates could indicate that factors linked to geographic area such as cultural habits including behavioral differences or environmental factors might mediate the association between sleep duration and overweight. Detailed cross-cultural research on factors that may interact with or complete existing biological as well as behavioral hypotheses for the association between sleep and weight is limited. Based on our data, regional affiliation did not seem to considerably modify this association, although an adjustment for region accounted for relevant changes in the effect estimate for sleep, even if additionally adjusted for environmental factors such as temperature in all models. This may be explained by factors that influence overweight and differ by country but are not associated with sleep duration and therefore were not included in our model (e.g., parental smoking habits32).

Age-stratified analysis showed a stronger association between sleep and overweight in school children than in preschool children and the relation seems to increase after transition from preschool to school. This contradicts a recent study that did not find any age dependency in the association between sleep duration and overweight.33

Based on a systematic review on the association between sleep duration and overweight in children, adolescents and adults, it has been suggested that the relationship between sleep duration and weight may weaken with age over the lifespan.7 Our data showed an increasing effect with age in children below the age of ten, indicating that the association may decrease later in life. Hence, it would be of interest to detect if and at which age this possible inversion of the effect size takes place. To date, data on the impact of age on the association between sleep and overweight in children is limited, calling for a more detailed investigation of this topic in younger age groups.

One of the strengths of our study is the large dataset of international data with a population based sample, including children from different age groups which allows for stratified analyses with sufficient statistical power. Age differences between children with sleep information and children without sleep information were significant, but small and therefore should not distort the association under investigation. Sleep duration in large scale studies can be most objectively measured by means of accelerometry, while parental report seems to overestimate the objectively measured sleep duration.1 However, we do not see any reason why this potential over reporting should be differential between parents of normal weight children as compared to parents of overweight children. A social desirability bias in reporting sleep-related data seems to be improbable because the possible association with overweight is not perceived by the general population. Last but not least, it may be mentioned that the study does not claim to be representative for the respective countries.

To the best of the authors' knowledge, this study is the first of its type to provide a large amount of comparable data from several European countries on the association between sleep duration and overweight in children. The results discussed above allow for the conclusion that geographic region and related factors—even if they did not seem to modify the association between sleep and overweight—should in any case be taken into consideration as a confounding factor for this association. The fact that sleep duration remained associated with overweight after adjustment for regional, behavioral and environmental variables—at least in school aged children—supports the hypothesis that a major part of the association may be explained by biological processes. To specify in more detail the cultural and environmental characteristics playing a role in this context, longitudinal internationally comparable data on these aspects will be needed.


This was not an industry supported study. The authors have indicated no financial conflicts of interest.


The authors sincerely thank the parents and children who participated in the study. This work was done as part of the IDEFICS Study (www.idefics.eu). The authors gratefully acknowledge the financial support of the European Community within the Sixth RTD Framework Programme Contract No. 016181 (FOOD).


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