Objectives Sleep problems are common in children with epilepsy. We aimed to evaluate the clinical effectiveness and incremental cost-effectiveness of an online resource for parents of children with epilepsy (CASTLE Online Sleep Intervention, or COSI) combined with standard care (SC) versus SC alone over six months.

Design We conducted a multicentre, parallel-group, unblinded, randomised controlled trial with an internal pilot.

Setting 26 UK NHS paediatric epilepsy outpatient clinics.

Participants Children aged 4–12 years with clinician-confirmed epilepsy and caregiver-reported sleep problems.

Interventions Participants were randomly assigned (1:1) via a computer-generated minimisation algorithm to SC or SC plus COSI.

Main outcome measures The primary outcome, assessed at three months, was the Children’s Sleep Habits Questionnaire (CSHQ). Cost-effectiveness was estimated at six months, with utilities from the Child Health Utility 9-Dimensions Index and costs from an NHS and Personal Social Services perspective. Safety analyses included all randomised participants. Analyses were performed on an intention to treat basis.

Results 85 children were enrolled between 30.08.2022 and 18.10. 2023, (42 SC; 43 SC+COSI). At three months, the adjusted mean CSHQ difference between SC+COSI and SC was 3.00 (95% CI 0.06–5.93; p=0.05), indicating no significant superiority. No adverse events were reported. Children in the SC+COSI group showed a mean 16.5-minute reduction in sleep onset latency and parents increased their knowledge. Only 23 (53%) families accessed the core online materials. Incremental mean cost of SC+COSI was £1,232 (95% credibility interval £535–£3,455) with a mean incremental QALY of 0.00 (95% CI -0.03 to 0.04), yielding an incremental cost-effectiveness ratio of £433,167 per QALY gained a (0.04 probability of being cost-effective at the £30,000/QALY threshold).

Conclusions An online Parent-Directed Behavioural Sleep Intervention did not reduce parent-rated sleep problems in children with epilepsy in the NHS context. However, improved objective sleep onset latency and enhanced parental knowledge suggest that the underlying behaviour change techniques have merit. Blended delivery through specialist nurses or similar support may improve engagement and outcomes.

Trial registration ISRCTN13202325.

Funding NIHR Programme Grant for Applied Research RP-PG-0615-20007.

The authors have declared no competing interest.

ISRCTN13202325.

This study was funded by National Institute for Health Research Programme Grant for Applied Research RP-PG-0615-20007.

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East Midlands Nottingham 1 ethics committee of the Health Research Authority, UK gave ethical approval for this work.

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