Is sleep duration related to obesity? A critical review of the epidemiological evidence

Summary

• Observational studies have implicated habitual sleep duration as a risk factor for mortality and morbidity. Part of this association might be mediated by obesity, which has also been associated with habitual sleep duration. These studies generate wide media attention because of the public's health concerns surrounding increasing obesity and the temporal association with the other modern “epidemic” of sleep loss. Some commentators have recommended public health interventions to control obesity via habitual sleep duration modification. We conducted a critical review of the available literature describing the relationship between habitual sleep duration and obesity in community-based studies in both adults and children, with particular emphasis on longitudinal studies and on studies with objective measures of habitual sleep duration.

• Existing data have variable consistency. Only one study objectively measured sleep duration for more than one 24-h period. Cross-sectional and longitudinal studies in adults often demonstrated an association of short sleep duration with BMI. However, some of these studies also showed that long sleep duration was also associated with obesity. In contrast, other studies showed that neither long nor short sleep was associated with obesity. In paediatric populations there appeared to be a clear pattern where shorter sleep durations were associated with obesity. We did not locate any interventional studies where sleep duration had been manipulated in order to prevent or treat obesity.

• We contend that the evidence base is not yet strong enough to give public health advice to the general population or specific groups about sleep duration being a modifiable risk factor for obesity. We need to experimentally clarify whether sleep duration variability is a risk factor for obesity, in what manner, and in which populations. If a reliable aetiological model could be found, we would ideally then need community-based randomised controlled trials that show that sleep duration can be changed and that sleep duration manipulation produces actual weight loss and/or prevents the development of obesity without undue side-effects.

Introduction

Recently, a number of epidemiological reports have provided evidence that habitual sleep duration is prospectively and independently associated with mortality4, 5, 33, 34, 35 and with common diseases such as type 2 diabetes12, 36 and heart disease.³⁷ The relationship is typically a U-shaped curve where the lowest risk is found at about 7–8 h of sleep per night with the odds rising for shorter and longer sleepers. There are, however, other studies reporting non-U-shaped associations between sleep duration and mortality (see Figure 1).7, 35, 38 As a result of these findings both sleep restriction³⁹ and sleep extension40, 41 have been suggested as potential health interventions.

However, controlling for obesity in these observational studies may be giving a false picture of cardiovascular disease (CVD) via ‘over-controlling’, if obesity is part of the causal chain through which sleep duration affects mortality. Indeed epidemiological papers are now regularly being published assessing the association of obesity with both long and short habitual sleep duration.

Although there are few proposed mechanisms for how longer sleep duration might be harmful, some experimental evidence exists to suggest short sleep may be causing morbidity. Short-term sleep restriction appears to impair glucose regulation and, if these effects continue over the long term, sleep restriction could also lead to obesity via changes in the hunger hormones leptin and ghrelin.42, 43, 44, 45 This journal has recently published a review⁴⁰ and editorial⁴¹ discussing the complex interactions between sleep and metabolism.

Reports of associations between short sleep duration and obesity have attracted widespread media attention⁴⁶ (BBC, CBS, WebMD FoxNews) and internet discussion. Front-page headings in Google when searching for ‘obesity and sleep’ include reputable sites such as http://sleepdisorders.about.com with ‘Less sleep, more weight’ and www.webmd.com with ‘Sleep more to fight obesity’. The US National Institutes of Health in 2006 issued a call for proposals for research into mechanisms for the link between short sleep and obesity (nih.gov; request for applications RFA-HL-06–003).

Given these enthusiastic media reports reporting the accumulation of scientific evidence for the association between sleep duration and obesity, we contend that it is timely to undertake a critical review of the key epidemiological literature underpinning the credibility of these internet-based health promotional messages. We would argue that the sleep research community has an inherent “conflict of interest” in promoting the concept that short sleep hours predispose to obesity and it is therefore important to discuss the strength of research findings in this area.