The metabolic syndrome (MetS) is routinely operationalised as a cluster of adverse risk factors for both cardiovascular heart disease and type 2 diabetes. Amongst the risk factors which may contribute to the development (and consequences) of MetS is sleep. The overarching aim of the present thesis is to generate an improved understanding of the available evidence regarding the speculative relationship between sleep and MetS. After a systematic review and meta-analysis of previous empirical studies exploring the relationship between MetS and sleep, the present study draws on a large-scale, nationally representative survey of UK adults in which directly measured and self-reported MetS symptoms/components and self-reported sleep characteristics have been recorded; and assesses the reliability of self-reported indicators of MetS before re-evaluating the association between MetS and sleep. The systematic review suggested that, while there is some evidence of an association between MetS and sleep across a range of sleep-related characteristics, this evidence draws on a small number of non-UK cross-sectional studies, some of which involved methods that are prone to error and bias. On the other hand, the self-reported MetS components identified in the UKHLS questionnaires provided substantial agreement with direct measures thereof. The subsequent associations observed between three key components of MetS (elevated waist circumference; high blood pressure; and diabetes – and different combinations thereof) and seven self-reported sleep characteristics were dependent upon: the specific sleep characteristic examined; the choice of referent group used; adjustment for sociodemographic and/or lifestyle covariates; and the use of self-reported or directly measured MetS components. Key differences between these findings and those of previous studies suggest these associations remain speculative and subject to methodological and contextual variation between studies.