Sleep problems are a common complaint in clinical samples. Specifically, the high level of co-occurring sleep disturbances, such as insomnia and hypersomnia, amongst patients with schizophrenia and bipolar disorder suggests a relationship between poor sleep and psychopathology. Furthermore, recent evidence consistently indicates that sleep disturbance may be related to symptoms of psychosis such as hallucinations and delusions. The research described in this thesis attempts to explore the relationship between sleep in psychosis. Paper 1 presents a systematic review entitled: Evaluating the Effectiveness of Sleep Treatments in Severe Mental Illness. The review identified fifteen controlled trials of pharmacological and non-pharmacological interventions for the treatment of sleep disturbance in patients with severe mental illness. The outcomes of the review indicated that sleep could be reliably improved in persons with schizophrenia and bipolar disorder. Significant improvements to sleep were observed for some pharmacological interventions as well as for non-pharmacological interventions. Paper 2 describes an empirical study aimed at exploring the effects of poor sleep on the cognitive mechanisms underlying psychotic experiences. Using an independent groups design, two, well-defined, non-clinical groups comprising good sleepers and poor sleepers (those meeting criteria for insomnia disorder) were recruited to the study. The two groups were compared on a series of questionnaires and computer tasks designed to assess mechanisms underlying hallucinations and delusions, with a view to determine whether there was a difference in performance between groups that could be attributed to sleep. No significant differences were found between groups, although the study was underpowered. The findings are discussed in the context of sample characteristics and tests used to compare groups. The final paper offers a critical reflection on the systematic review and empirical study, drawing together conclusions about the role and treatment of sleep in relation to psychotic experiences.