Introduction Cognitive-behavioural therapy (CBT) should be the first-line treatment for insomnia disorder but access remains extremely limited. We aimed to assess the clinical effectiveness of an abbreviated version of CBT - brief sleep restriction therapy (SRT) - which has the potential to be widely implemented. We also assessed potential treatment moderators and mediators, and the association between treatment outcome and SRT engagement (defined as session attendance and diary-defined adherence to prescribed bed and rise-times).

Methods We did a pragmatic, individually randomised, superiority, open-label trial of SRT versus sleep hygiene. Adults with insomnia disorder were recruited from general practice in England and randomised to either four-session nurse-delivered SRT plus sleep hygiene booklet, or sleep hygiene booklet only. There was no restriction on usual care for either group. Outcomes were assessed at 3, 6, and 12 months. The primary endpoint was self-reported insomnia severity at 6 months measured with the insomnia severity index (ISI). The trial was prospectively registered (ISRCTN42499563) and analyses were pre-specified.

Results We randomised 642 participants to SRT (n=321) or sleep hygiene (n=321). Mean age was 55 years and 76% were female. At 6 months, the estimated adjusted mean difference on the ISI was -3·05 (95% CI: -3·83 to -2·28, p<0·0001; Cohen’s d=-0·74) in favour of SRT. Baseline demographic and clinical variables (depression, sleep medication use, chronotype, age, level of deprivation, and actigraphy-defined sleep duration) did not moderate the treatment effect. Pre-sleep cognitive and somatic arousal and sleep effort were significant mediators of the treatment effect (proportion mediated: 15–36%). Attending more SRT sessions and more closely adhering to prescribed bed and rise-times were associated with larger treatment effects.

Discussion Brief nurse-delivered SRT improves insomnia symptoms and has the potential to be widely implemented as first-line treatment in primary care.