Sleep exerts important modulatory effects on neuroendocrine function and glucose regulation. During the past few decades, sleep curtailment has become a very common behavior in industrialized countries. This trend toward shorter sleep times has occurred over the same time period as the dramatic increases in the prevalence of obesity and diabetes. This article will review rapidly accumulating laboratory and epidemiologic evidence indicating that chronic partial sleep loss could play a role in the current epidemics of obesity and diabetes. Laboratory studies in healthy young volunteers have shown that experimental sleep restriction is associated with a dysregulation of the neuroendocrine control of appetite consistent with increased hunger and with alterations in parameters of glucose tolerance suggestive of an increased risk of diabetes. Epidemiologic findings in both children and adults are consistent with the laboratory data.