The present thesis addresses two issues. First, little research has explored the potential of targeting sleep difficulties to reduce the impact of symptoms of psychosis. Second, while self-help interventions are widely used for common mental health problems and sleep difficulties, they are underdeveloped with respect to psychosis. Therefore, the present thesis investigates the potential of sleep difficulties, specifically insomnia, as a target for self-help interventions designed to reduce an experience that is often associated with psychosis; namely, paranoia. Meta-analysis was used in Chapter 3 to quantitatively synthesise the efficacy of extant self-help interventions for psychosis. Following this, Chapter 4 explored the link between sleep difficulties and paranoid thinking using structural equation modelling and multiple regression. Finally, Chapter 5 describes a randomised controlled trial designed to investigate the impact of an online, self-help intervention for insomnia on paranoid thinking. Chapter 3 revealed that, on average, self-help interventions have a small-to-medium sized effect on the symptoms of psychosis and a small-sized effect on associated outcomes. Chapter 4 found that self-reported delayed sleep onset, but not sleep maintenance, was associated with increased paranoia. Further analysis revealed that this relationship was fully mediated by negative affect. Chapter 5 found significant reductions in insomnia, negative affect and paranoia in those who received the self-help intervention when compared to those in the wait-list control group immediately post-intervention. The research described in this thesis suggests that; (i) self-help interventions are a viable intervention option for experiences of psychosis; (ii) sleep difficulties are associated with negative affect and, in turn, the experience of paranoia; and (iii) sleep difficulties are a possible target for self-help interventions designed to reduce paranoid thinking.