Original ArticleThe association between short sleep and obesity after controlling for demographic, lifestyle, work and health related factors
Introduction
One-third of North American adults are reported to be obese (body mass index [BMI], ⩾ 30 kg/m2) and the prevalence rate has tripled over the last three decades [1]. Similarly, Australian estimates suggest 29% of its population will be obese by 2025 [2]. The development of obesity is multi-causal, but the link between short sleep duration and BMI has received a good deal of attention with suggestions that there are direct and indirect links between short sleep and obesity [3]. A meta-analysis of 18 cross-sectional studies reported a pooled odds ratio (OR) of 1.55 between short sleep and obesity [4]. However, not all cross-sectional studies have reported this relationship [5], and the results obtained from longitudinal studies also report mixed results [6], [7], [8].
One possible explanation for the inconsistent findings in the literature is the absence of sufficient control variables that may confound the sleep–obesity relationship [9], [10], [11]. A review of 13 longitudinal studies found that six studies adjusted for education, employment, and gender; seven for the influence of alcohol; five for sleep concerns; nine for physical activity; two for diet; and eight for smoking [9]. Work-related factors are another set of variables that often are not considered. For example, shift work is a prime reason for obtaining insufficient sleep [12] but few studies have controlled for night work [11], [13], [14]. Other studies have suggested that longer work hours have a stronger association with obesity [15] than with shift work [13].
The possible confounders in the sleep–obesity relationship extend beyond work-related factors. Thomas et al [11] demonstrated a link between short sleep and BMI when controlling for demographic, health behaviour, and work-status variables. However, this relationship was not significant when physical and emotional health variables were added to the model. Depression and emotional health appear to be more strongly associated with BMI than is short sleep, but most studies have not controlled for these factors [8], [11], [16]. Furthermore, the literature assumes short sleep duration is a choice and has paid less attention to the role of sleep disorders [17] and hypercholesterolemia [18] on sleep duration. Finally, evidence suggests short sleep and obesity also may be related to the amount of time spent watching television and using social media [19], [20].
In summary, the relationship between sleep and obesity may be obscured by the inclusion of too few confounding variables. In our study, we assessed the influence of a larger number of variables to provide a more robust assessment of the association between short sleep and obesity.
Section snippets
Methods
Cross-sectional self-report data were obtained via a telephone survey administered in three regional cities in central Queensland, Australia. Participants completed a number of questions and scales as part of a larger study on the health of shift workers. The study was approved by the Human Research Ethics Committee at Central Queensland University.
Random digit dialling was used to contact 2323 households with a landline telephone. Interviewers asked to speak to an adult (⩾18 years) who was
Data analyses
Statistical analyses were conducted using SPSS 20. A number of increasingly complex multinomial logistic regression models were applied to assess the relationship between the predictor variables and BMI and were categorised into the following categories, normal, overweight, and obese. A bivariate model assessed the simple relationship between sleep and BMI (model 1). The subsequent models controlled for the effect of the confounding variables by sequentially adding demographic factors (model
Results
Overall 1162 interviews were completed (50% response rate). The sample comprised 622 women (53.5%) and 540 men (46.5%). Mean age was 45.27 ± 11.23 years and did not significantly differ by gender. Additional details describing the sample can be found in Table 1.
The bivariate multinomial model obtained a significant association between short sleep and obesity but not with being overweight (model 1). The inclusion of additional control variables produced a consistent result; short sleep was
Discussion
Recent studies have suggested that the inconsistent findings in the relationship between short sleep and obesity may be attributed to the fact that too few confounding variables have been considered [9], [10], [11]. We directly addressed this limitation and adjusted for the role of 17 variables; the results from our models support a significant association between short sleep and obesity, but not between short sleep and being overweight.
The OR suggested the strongest associations with obesity
Conflict 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.2012.12.007.
Acknowledgments
The Health and Shift Work Study was funded by an internal grant from the Institute for Health and Social Science Research and conducted by the Population Research Laboratory at Central Queensland University.
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