The prevalence of anxiety disorders, and sleep disturbance is elevated among autistic youth. However the ways in which anxiety, autism characteristics, and sleep disturbance interact with each other, in the context of an individual’s environment and family relationships is still unclear. The reciprocal influences between these variables is diverse, and multi-faceted. Despite the relatively common co-occurrence of sleep disturbance and anxiety in autistic and non-autistic individuals, these conditions continue to be treated separately, with minimal evidence of effective integrated treatment for anxiety related sleep disturbance. A greater understanding of the bi-directional relationship between anxiety and sleep in autistic individuals, and the development of minimally sufficient treatment of anxiety-related sleep disturbance is needed. This study evaluated the effectiveness of Functional Behavioural Assessment (FBA) - informed behavioural sleep interventions for the treatment of anxiety related sleep disturbance, utilising single-case-research design. Study One is presented as a case study with one 13-year-old participant. Study Two is a single-case multiple-baseline AB design involving three participants aged 13-16 years. The effects of sleep treatment on children’s anxiety, daytime behaviour, and quality of life, as well as parents’ sleep quality and well-being was examined. All four participants were adolescents, with a diagnosis of Autism, and parent or self-reported sleep-interfering anxiety. Participants’ interventions were informed by FBA, and individualised according to the needs of the child and their family. In Study One, treatment included relaxation strategies and faded parental presence procedures. Study Two included adjustment of participants’ sleep/wake cycle for all participants, with one participant also requiring sleep hygiene modifications. Results indicated substantial improvements in sleep for all participants who completed treatment, with improvements largely maintained up to 10 weeks following treatment. Findings related to anxiety were variable, with some indication of both reductions and increases in perceived anxiety following treatment. Positive changes were evident overall in participants’ daytime behaviour, quality of life, and parent well-being post-treatment, with mixed results relating to parents’ sleep quality. Parents and participants indicated that treatment was acceptable, cost effective, reasonable, and understandable. Families found treatment time-consuming, but worthwhile due to the benefits of effective treatment, and families felt well-supported by clinicians throughout the treatment process. Overall, this study provides evidence of the effectiveness of behavioural sleep interventions for treating sleep disturbance in anxious autistic youth, and highlights the utility of FBA informed interventions for providing individualised, responsive treatments, uniquely tailored to the needs of autistic individuals and their family.