1. 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. 2. 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. 3. 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.