Since its early development the concept of perseverative thinking or cognition (PC, i.e., rumination, worry) was established as one of the most consistent transdiagnostic factors of mental (e.g., depression) and physical (e.g., cardiovascular) disorders. In the context of behavioral sleep medicine, PC was traditionally introduced as a maintaining factor for sleep onset insomnia through increased physiological (e.g., cortical, autonomic, neuroendocrine) arousal. Recent evidence, though, also suggests that sleep may modulate PC itself. Moreover, experiencing PC in individuals with insomnia may be associated with affective symptoms and increased depressive mood throughout the day. Reversely, behavioral sleep interventions may moderately reduce PC and improve mood in adult individuals with insomnia. In this chapter, we will describe the following: (1) the concept of physiological arousal and its biomarkers in the study of insomnia-depression associations; (2) currently available strategies, procedures, and tools to assess PC in the context of insomnia and depressive disorders; and (3) available psychological techniques targeting PC in patients with insomnia and/or depression, including traditional cognitive therapy and third wave cognitive behavioral therapy approaches.