Disruption of circadian rhythms negatively affects regulation of metabolism and energy homeostasis. Disrupted metabolism in response to disrupted biological rhythms might lead, together with genetic background, to obesity and to other health complications. Results of epidemiologic surveys are consistent with mechanistic theory showing the interconnection between the biological rhythms, sleep and metabolism. Epidemiologic surveys confirm that sleep duration of less than 6 hours increases significantly the risk of obesity. Systematic reviews of epidemiologic surveys examining association of sleep and obesity refer to large heterogeneity in involved subjects, methodological approaches of measuring obesity and sleep, and confounders. Design of study plays also essential role in interpretation and definition of causal relationship. Reduced sleep duration in relation to obesity is in the literature discussed from different points of view: 1. as a possible primary cause of obesity, 2. as a result of comorbidities resulting from obesity and 3. as an accompanied part of the third factor contributing to obesity (e.g. long working hours, chronic emotional stress, overusing of media). Causal relationship between sleep and obesity is not yet fully elucidated, however the association is supposed to be bidirectional. The article gives an overview of current knowledge concerning the influence of sleep on the development of obesity and points to the critical points of current research.