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Discrepancies between sleep diaries and actigraphy occur among individuals with insomnia. Cognitive behavioural therapy for insomnia (CBT-I) improves insomnia but the impact on discrepancy is unclear. This study examined CBT-I's effects on actigraphy-diary discrepancy and explored sleep-related beliefs and attitudes as a mediator. Participants were 108 (age M ± SD = 47.23 ± 12.42, 67.60 % female) adults with insomnia and major depressive disorder from the Treatment of Insomnia and Depression study. They were randomized to 7 sessions of CBT-I or sham Quasi-Desensitization Therapy for Insomnia (DTI), plus 16 weeks of antidepressants. Two weeks of actigraphy and sleep diary were collected at baseline, mid-treatment, end-treatment. Differences between sleep diary and actigraphy total sleep time (TST), sleep onset latency (SOL), wake after sleep onset (WASO), and sleep efficiency (SE) were calculated. Participants completed Dysfunctional Beliefs and Attitudes about Sleep Scale (DBAS) at baseline and mid-treatment. At baseline, diary (versus actigraphy) TST was shorter (1.1 ± 1.41h), whilst SOL (21.64 ± 41.25min) and WASO (17.45 ± 61.99min) were longer. Mixed effects models using daily data showed that after adjusting for age and sex, participants in the CBT-I group (versus DTI) showed greater reduction in all actigraphy-diary discrepancy domains (all p-values <.01), reductions evident from mid-treatment. Group differences on actigraphy-diary discrepancy reductions in TST, SOL, and SE (not WASO) were mediated by changes in DBAS from baseline to mid-treatment (all p-values <.05). Changes in discrepancy did not mediate insomnia symptom changes (p-values >.39). CBT-I reduced actigraphy-diary discrepancy in individuals with comorbid insomnia and depression; this reduction was associated with improved sleep-related attitudes, a therapeutic target of CBT-I. TRIAD (Treatment of Insomnia and Depression): Improving Depression Outcome by Adding Insomnia Therapy to Antidepressants. Prospectively registered with Clinical Trials (NCT00767624). MH078924, MH078961, MH079256. • Individuals with insomnia and depression reported shorter and worse sleep compared to actigraphy measures. • CBT-I reduced actigraphy-diary discrepancy in these individuals. • Discrepancy reduction was associated with improved sleep-related attitudes and beliefs. [ABSTRACT FROM AUTHOR]