Sleep disorders, sleep fragmentation, and chronically reduced sleep duration are increasingly common in western societies. In parallel, incidence of the metabolic syndrome and its key components, i.e. type 2 diabetes and obesity, is rapidly increasing. A huge number of epidemiological studies has shown a robust association between disturbed sleep quality, reduced sleep duration and the development of components of the metabolic syndrome. Moreover, there is growing evidence from experimental studies proving a causal link between sleep loss and disturbed human energy homeostasis. Short term sleep loss has been shown to reduce insulin sensitivity and glucose tolerance, increase feelings of hunger by modulating orexigenic/anorexigenic hormonal signaling, and disturb physical activity behavior. This review attempts to present an overview of the presently available literature on the link between sleep loss and disturbed human energy homeostasis, as well as on potential pathophysiological mechanisms.