The study aimed to investigate a group of children with severe learning disabilities, challenging daytime behaviour, and severe sleep problems to see if successful behavioural treatment of the children's sleep problems resulted in reduced daytime challenging behaviour as reported by mothers and teachers. A randomised controlled trial of behavioural interventions for the children's sleep problems was conducted (N = 30). The intervention group received an individually tailored behavioural programme and were supported by telephone calls from the therapist. Baseline assessments of the children's behaviour were made using the Aberrant Behaviour Checklist and were repeated 1 month and 3 months after the start of the intervention. There were no behavioural changes that were specific to the children in the treatment group. However, improvements in some behaviours were seen in both the intervention and the control groups at the 1-month and 3-month assessments, and there was agreement between mothers' and teachers' reports for many of these changes. The results suggest that nonspecific effects of participating in the study (including an increased sleep duration, which was seen in both groups), rather than resolution of sleep problem per se, may have a beneficial effect on child behaviour and these factors need to be identified for therapeutic use.