Invited Commentary: understanding the role of sleep.

Chronic sleep deprivation is increasingly entertained as a novel risk factor for obesity. However, the vast majority of studies on this topic have relied on unvalidated subjective measures of habitual sleep habits. The accompanying paper by Lauderdale et al. (Am J Epidemiol. 2009;170(7):805-813) presents the first longitudinal analysis of the relation between sleep duration and weight change by using an objective assessment of sleep. The lack of evidence for an association in this work suggests that the absolute time slept may not be important for weight regulation but raises questions as to what self-reported sleep duration is measuring. One intriguing possibility is that self-reported sleep may reflect the time spent in deeper stages of sleep, which physiologic studies suggest may be more relevant from a metabolic standpoint. Further research into the relation between sleep quantity and quality relative to obesity by use of more refined measures of sleep is needed to identify which, if any, aspects of sleep are important in weight homeostasis.