Sleep problems are very common in people with schizophrenia spectrum diagnoses, they cause considerable distress, and they impact negatively on quality of life, physical and mental health, and social functioning. Few non-pharmacological treatments are offered in practice, and therapies tested thus far tend to focus on insomnia processes more than on circadian dysregulation. Both insomnia and circadian dysregulation commonly occur in people with schizophrenia spectrum disorders. The presentation of sleep problems in this group is diverse, and social and occupational factors appear to contribute. It is suggested that occupational therapists have relevant skills to address such sleep problems. This thesis presents work to develop an acceptable, safe and user-centred behavioural treatment for poor sleep, in people with schizophrenia spectrum disorders, to be delivered by an occupational therapist. This is the first programme of research that we are aware of that is pursuing this aim. Studies A and B evaluate existing evidence regarding interventions altering light exposure patterns to improve sleep in groups with high rates of circadian dysregulation. They present a meta-analysis with meta-regression, and a narrative synthesis of acceptability and adherence data. They describe evidence of positive effects of light exposure modifications on sleep, and areas where more research is needed. Despite many light therapy protocols appearing insufficiently flexible, acceptability appears to be high, and attrition low. Study C is a mixed methods expert opinion study to support intervention development, based on the principles of Delphi methodology. Expert opinion emphasised the need for treatment personalisation, and this is reflected in the intervention developed. Study C is followed by a chapter describing the process of synthesis of background evidence and findings from Studies A-C, describing how these informed the intervention. Study D surveys service users and staff, to identify potential implementation issues, including the demand for such an intervention, its acceptability, and barriers to referral. Findings identified significant demand for such an intervention and suggested increased staff training could improve sleep problem identification and referral. Finally, the protocol for feasibility testing of this intervention is presented. This mixed methods study evaluates both the acceptability of the intervention, and the feasibility of larger scale testing, and at the time of writing it is underway.