Original ArticleHabitual sleep patterns and the distribution of body mass index: cross-sectional findings among Swedish men and women
Introduction
Between 1980 and 2008, worldwide obesity prevalence nearly doubled from 5% to 10% in men and from 8% to 14% in women [1]. Alongside the obesity epidemic, sleep loss also has become more prevalent, as reflected in the increased percentage of people sleeping ≤6 h [2]. Hence, research into the possible link between sleep loss and obesity is mounting.
Several cross-sectional studies have reported an association between self-reported short sleep duration and higher body mass index (BMI, weight in kilograms divided by height in meters squared) or increased obesity risk [3], [4]. Whether sleep duration is related to weight gain, however, is not clear. Prospective studies, from which inferences on the direction of possible causality may be drawn, have yielded mixed results [5]. The literature on sleep quality or sleep disturbances in relation to body weight is less extensive, but studies have demonstrated that worse self-reported sleep quality or sleep disturbances are positively linked to obesity and weight gain [6], [7], [8].
Most of the previous studies analyzed the association between sleep and body weight measures using linear or logistic regression models to estimate mean BMI or the probability of obesity, respectively. Associations of sleep patterns with other parts of the BMI distribution have rarely been explored. Quantile regression, which allows for evaluation of predictors across the entire distribution of an outcome variable, such as BMI, has only been used in two studies on the relationship between sleep and body weight in adult populations. Both studies indicated that the distribution of BMI differed according to level of habitual sleep duration, so that shorter sleep was associated with higher values of BMI at the upper part of the distribution, particularly in men [9], [10]. Better sleep quality was associated with lower values of BMI at the lower part of the BMI distribution in women only [10]. Limited evidence thus suggests that the association with short or poor sleep varies across the distribution of BMI. Similar studies have not been conducted in European populations.
Analyses taking account of the entire distribution of BMI have the potential of providing a more comprehensive picture of the nature of the association between sleep and body weight. They also offer an opportunity to identify subgroups of the population that could be especially susceptible to the putative effects of poor sleep habits. Those subgroups would likely not be identified by approaches analyzing the influence of sleep habits on mean BMI [11]. Furthermore, the vast majority of studies within this research area have focused on sleep duration, typically using one sleep question. There are considerably fewer data on sleep quality and other potentially relevant dimensions of sleep in relation to body weight.
Our aim was to compare the entire distributions of BMI between people with different habitual sleep patterns using quantile regression as analytical approach in a large sample of Swedish men and women. Self-reported sleep assessments were wide-ranging and included sleep duration separately for weekdays and weekends, sleep quality, restorative power of sleep, and daytime sleepiness.
Section snippets
Study population and setting
We used data from the National March Cohort, established in September 10–14, 1997 as a cohort of volunteers who took part in the National March, a nationwide promotional and fundraising event for the Swedish Cancer Society [12]. Participants in this event, which included a voluntary walk for cancer, were invited to fill out a 36-page questionnaire on lifestyle and medical history. Walking the walk was not a prerequisite for study participation. The Regional Ethics Review Board at the Karolinska
Descriptive characteristics of the study population
Table 1 summarizes the characteristics of the study population according to sex and sleep duration on weekdays. Of the total sample, approximately 8% were short sleepers (≤5 h) and 2% long sleepers (≥9 h). These participants were less educated, less physically active, and more likely to have high-frequency alcohol consumption, nonrestorative sleep, and daytime sleepiness than medium-length sleepers (6–8 h). Obesity was more common in male short sleepers, and in female short and long sleepers.
Discussion
This cross-sectional study is one of the first to examine associations across the entire distribution of BMI within sleep–body weight research. The distribution of BMI was extended toward higher values in men and women with short sleep duration. Women with long or poor-quality sleep also showed higher BMI values at the upper part of the distribution, whereas men with long sleep displayed lower BMI values at the lower tail. Overall, differences at the middle part of the BMI distributions were
Funding
None.
Conflicts of interest
The ICMJE Uniform Disclosure Form for Potential Conflicts of Interest associated with this article can be viewed by clicking on the following link: http://dx.doi.org/10.1016/j.sleep.2014.06.012.
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