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SummaryBackgroundSleep difficulties might be a contributory causal factor in the occurrence of mental health problems. If this is true, improving sleep should benefit psychological health. We aimed to determine whether treating insomnia leads to a reduction in paranoia and hallucinations.MethodsWe did this single-blind, randomised controlled trial (OASIS) at 26 UK universities. University students with insomnia were randomly assigned (1:1) with simple randomisation to receive digital cognitive behavioural therapy (CBT) for insomnia or usual care, and the research team were masked to the treatment. Online assessments took place at weeks 0, 3, 10 (end of therapy), and 22. The primary outcome measures were for insomnia, paranoia, and hallucinatory experiences. We did intention-to-treat analyses. The trial is registered with the ISRCTN registry, number ISRCTN61272251.FindingsBetween March 5, 2015, and Feb 17, 2016, we randomly assigned 3755 participants to receive digital CBT for insomnia (n=1891) or usual practice (n=1864). Compared with usual practice, the sleep intervention at 10 weeks reduced insomnia (adjusted difference 4·78, 95% CI 4·29 to 5·26, Cohen's d=1·11; p<0·0001), paranoia (−2·22, −2·98 to −1·45, Cohen's d=0·19; p<0·0001), and hallucinations (−1·58, −1·98 to −1·18, Cohen's d=0·24; p<0·0001). Insomnia was a mediator of change in paranoia and hallucinations. No adverse events were reported.InterpretationTo our knowledge, this is the largest randomised controlled trial of a psychological intervention for a mental health problem. It provides strong evidence that insomnia is a causal factor in the occurrence of psychotic experiences and other mental health problems. Whether the results generalise beyond a student population requires testing. The treatment of disrupted sleep might require a higher priority in mental health provision.FundingWellcome Trust.

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People with bipolar disorder and comparatively fewer previous mood episodes may benefit from CBT. However, such cases form the minority of those receiving mental healthcare.

Causal attributions for positive and negative hypothetical social events made by paranoid patients, depressed patients, and nonpatient participants were examined via a novel measure of causal locus, the Internal, Personal and Situational Attributions Questionnaire. Depressed patients tended to attribute negative social events to internal (self-blaming) causes. Nonpatient participants and patients with delusions of persecution tended to avoid such self-blame. However, whereas nonpatient participants tended to choose situational or circumstantial external attributions, paranoid patients tended to choose external attributions that located blame in other individuals. These findings support R. P. Bentall, P. Kinderman, and S. Kaney's (1994) defensive attributional model of persecutory delusions, suggest some modifications to that model, and have implications for the understanding of the relationship between causal attributions and social and self-perception.