This study examined the sustained and moderating effects of a behavioural sleep intervention for autistic children in a randomised controlled trial. Autistic children (5-13 years) with sleep problems were randomised to the Sleeping Sound intervention or Treatment as Usual (TAU). At 12-month follow-up (n = 150), caregivers of children in the Sleeping Sound group reported greater reduction in child sleep problems compared to TAU (p < 0.001, effect size: - 0.4). The long-term benefits of the intervention were greater for children taking sleep medication, children of parents who were not experiencing psychological distress, and children with greater autism severity. The Sleeping Sound intervention demonstrated sustained improvements in child sleep. Identified moderators may inform treatment by indicating which subgroups may benefit from further support.