LAUSR

There is a strong relationship between the symptoms of insomnia and depression, however, little is understood about the factors that mediate this relationship. An understanding of these underlying mechanisms may inform the advancement of existing treatments to optimise reductions in insomnia and depression when they co-occur. This study examined rumination and unhelpful beliefs about sleep as mediators between symptoms of insomnia and depression. It also evaluated the effect of cognitive behavioural therapy for insomnia (CBT-I) on rumination and unhelpful beliefs about sleep, and whether these factors mediated the effect of CBT-I on depressive symptoms. A series of mediation analyses and linear mixed modelling were conducted on data from 264 adolescents (12-16 years) who participated in a two-arm (intervention vs. control) randomised controlled trial of Sleep Ninja®, a CBT-I smartphone app for adolescents. Rumination, but not unhelpful beliefs about sleep, was a significant mediator between symptoms of insomnia and depression at baseline. CBT-I led to reductions in unhelpful beliefs about sleep, but not in rumination. At the between-group level, neither rumination, nor unhelpful beliefs about sleep emerged as mechanisms underlying improvement in depression symptoms, however, rumination mediated within-subject improvements following CBT-I. The findings suggest rumination links symptoms of insomnia and depression and provide preliminary evidence that reductions in depression following CBT-I occurs via improvements in rumination. Targeting rumination may improve current therapeutic approaches.