Objective: Insomnia and anxiety often co-occur. Having both disorders is associated with more severe symptoms and greater functional impairment. This study examined whether Sleep Restriction Therapy (SRT), a core component of Cognitive Behavioural Therapy for Insomnia (CBT-I), reduces anxiety symptoms in individuals with insomnia and whether improvements in sleep efficiency (SE) and reductions in pre-sleep arousal mediate this effect.

Methods: In a randomized controlled trial, 147 adults insomnia were assigned to either a six-week telephone-guided SRT intervention or a sleep diary control condition. Anxiety symptoms (HADS-A) were measured pre- and post-treatment. Sleep efficiency and pre-sleep arousal (PSAS) were assessed at pre-, mid-, and post-treatment. Multilevel modelling and bootstrapped mediation analyses were used to evaluate treatment effects and mediators.

Results: Participants receiving SRT showed a significantly greater reduction in anxiety symptoms than those in the control group. SRT also led to significant improvements in SE and reductions in pre-sleep arousal. Mediation analyses revealed that reductions in pre-sleep arousal, but not SE, significantly mediated the effect of SRT on anxiety. The indirect effect via pre-sleep arousal accounted for 63% of the total treatment effect.

Conclusion: SRT reduced anxiety symptoms in individuals with insomnia, . The study also highlights arousal as a mediator underlying the relationship between sleep and anxiety. Given its scalability and remote delivery format, SRT may be a valuable intervention for addressing comorbid insomnia and anxiety in both clinical and preventive contexts.