Sleep and circadian rhythm disturbance are increasingly recognized in anxiety-related disorders; however, the links between sleep and circadian rhythms and specific transdiagnostic processes evident in these disorders and their treatment have not been delineated. The present study examined the associations between chronotype, insomnia symptoms, and repetitive negative thinking (RNT) using a two-study approach. Study 1 examined concurrent and prospective associations between chronotype, insomnia symptoms, and RNT in a community sample. Study 2 then examined change in RNT following cognitive behavior therapy for insomnia (CBTI). The findings of Study 1 showed that evening chronotype and higher insomnia symptoms were concurrently associated with higher RNT in the community sample; however, only eveningness prospectively predicted increased RNT over 5 months, controlling for depression symptoms. Study 2 found that RNT decreased following CBTI, and decreased eveningness and insomnia symptoms were associated with decreased RNT, controlling for change in depression symptoms. Together these findings highlight the role of sleep and circadian rhythms in RNT. The implications for conceptualizing the etiology and treatment of anxiety-related disorders characterized by RNT are discussed.