This qualitative study develops a programme theory demonstrating the complexity embedded in sleep hygiene education (SHE) as an intervention to improve sleep problems in children with developmental disabilities. In co-design workshops, eight parents and six sleep practitioners deliberated themes developed from findings of an earlier exploratory study of stakeholder perceptions of SHE. A SHE tool underpinned by programme theory was developed evidenced by mid-range theories of change. Analytical themes were developed to explain the programme theory and the complexities of a successful SHE intervention: the need to legitimize children's sleep problems and consider the nature of customization, knowledge sharing, health expectation and impact of sleep service rationing and gaming strategies on implementation success. Policy and practice implications include a need to raise the public profile of children's sleep problems and promote parental involvement in intervention implementation. Further research is needed to test out this theory-driven framework for evaluating SHE.