| Summary: | <p>Sleep dysfunction – including sleep disorder symptoms, dissatisfaction with sleep, and other sleep abnormalities - has long been linked with psychosis, but the traditional view has been that sleep dysfunction is a consequence of psychosis. This thesis reconceptualises this relationship by testing a causal role of sleep dysfunction in psychosis. The first step was a systematic review (Chapter 1), in which correlational, but no causal, evidence is found for an association between sleep dysfunction and psychosis. This was addressed in Chapter 2 with a non-clinical manipulation study (n=68) which found that sleep loss caused increases in psychotic experiences, mediated by increases in negative affect. Chapter 3 explored the longitudinal relationship between short sleep duration and psychotic experiences in an at-risk group (n=160), finding that short sleep duration predicted later severity of psychotic experiences (although this prediction was not robust against conservative controls). In Chapter 4 the first detailed assessment of sleep disorders in a clinical group of individuals with psychosis (n=60) was reported, finding that sleep disorders were common, untreated, and often complex. A subgroup (n=29) were assessed twice more over the subsequent three months, allowing statistical modelling of the influence of insomnia on psychotic experiences over time (Chapter 5). The results again supported a significant predictive role of insomnia on psychotic experiences in this clinical group, mediated by negative affect. Finally, a novel exploratory study into hypersomnia in psychosis (n=26; Chapter 6) was carried out. The results suggested that hypersomnia is not associated with sedation or longer sleep duration, but is associated with increased fatigue, sedentary behaviour and depression. Overall, this thesis demonstrates that sleep dysfunction is not just a consequence of psychosis, but appears to have a causal role in the onset and maintenance of psychosis. The clinical implications are that sleep might be a novel therapeutic target in treating psychosis.</p>
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