The metabolic syndrome represents a clustering of several interrelated risk factors of metabolic origin that are thought to increase cardiovascular risk. It is still uncertain whether this clustering results from multiple underlying risk factors or whether it has a single cause. One metabolic abnormality that may underlie several clinical characteristics of the metabolic syndrome is insulin resistance. This review discusses the evidence that sleep disturbances (obstructive sleep apnoea, sleep deprivation and shift work) may independently lead to the development of both insulin resistance and individual clinical components of the metabolic syndrome. The converse may also be true, in that metabolic abnormalities associated with the metabolic syndrome and insulin resistance may potentially exacerbate sleep disorders. The notion that sleep disturbances exert detrimental metabolic effects may help explain the increasing prevalence of the metabolic syndrome and insulin resistance in the general population and may have important implications for population-based approaches to combat the increasing epidemic of metabolic and cardiovascular disease.