Insufficient sleep, which has become endemic in recent years, has been variably associated with increased risk of obesity, disorders of glucose and insulin homeostasis, and the metabolic syndrome; to a lesser degree, so has excessive sleep. This review summarizes recent epidemiological and pathophysiological evidence linking sleep disturbances (primarily abnormalities of sleep duration) with obesity, insulin resistance, type 2 diabetes and the metabolic syndrome in children and adults. •Insufficient sleep is common; 33% of adults and a majority of adolescents report sleeping less than the recommended duration.•Extensive epidemiological evidence links insufficient and, in some studies, excessive sleep duration with obesity•Insufficient and to a lesser degree prolonged sleep duration may predispose to type 2 diabetes and the metabolic syndrome