Mood instability (MI) and sleep disturbance (SD) are common in the general population, especially among persons with mental illness. Independently, MI and SD are associated with a range of poor health outcomes; however, there exists little evidence about their relationship. Using a series of six studies, this thesis investigated the relationship between MI and SD as transdiagnostic features on a shared pathway to ill health. A systematic review of the prevalence of MI in the general population showed few and highly heterogeneous reports available within the literature. The results of the review highlight pervasive challenges underlying the study of MI, particularly the absence of consistent definitions and thoroughly validated measurement tools. To investigate the epidemiological association between MI and SD, studies two and three examined data from a nationally representative survey of English adult household residents. Study three revealed that both MI and SD are common and closely linked within the population and associated with increased occurrence of suicidal ideation and behaviour. Study four showed that MI and SD are strongly and significantly associated with a wide range of poor mental and physical health and social outcomes such as difficulties with activities of daily living, socioeconomic indicators, and diagnosed health conditions. In study four, prospective data on MI and SD from electronic health records from secondary mental health care showed that MI and SD are associated with significantly higher rates of mental health services use, hospitalisation, and prescribed medication. Finally, an attempt to disentangle the directionality between MI and SD found strong cross-sectional evidence, but weak evidence of temporal trends between mood and sleep changes. Taken together, the results of this thesis highlight existing and ongoing challenges to the study of highly temporal data on a large scale. However, the findings of this thesis provide essential knowledge for linking future epidemiological studies with high resolution examination of dynamic processes such as MI. Clinically, there is a critical need for increased recognition of MI and SD as modifiable risk factors of illness and related complications, rather than secondary symptoms. Earlier and effective intervention may help to reduce the burden associated with MI and SD, and mental illness.