Preliminary support for the role of reward relevant effort and chronotype in the depression/insomnia comorbidity

Background: The presence of insomnia in the context of depression is linked to a number of poor outcomes including reduced treatment response, increased likelihood of relapse, and greater functional impairment. Given the frequent co-occurrence of depression and insomnia, research into systems and processes relevant to both disorders, specifically reward processing and circadian rhythm disruption, may help parse this complex comorbidity. Methods: A pilot study was conducted on a sample of 10 veterans with clinically significant depression and insomnia symptoms. Participants completed objective (actigraphy) and subjective (sleep diary) assessments of sleep, self-reports of chronotype, and behavioral tasks assessing reward relevant effort before and after 6 sessions of Cognitive Behavioral Therapy for Insomnia. Results: Insomnia and depression significantly improved following CBT-I. Subjective sleep parameters significantly improved with large effect sizes. Actigraphy results were nonsignificant, but effect sizes for sleep efficiency and onset latency were in the medium range. Chronotype shifted significantly toward morningness following CBT-I, and an earlier chronotype at baseline was associated with increased reward effort following treatment. Changes in chronotype, depression and insomnia were not associated with changes in effort. Limitations: Findings are limited by small sample size and lack of randomized control group. Conclusions: Findings should be interpreted as hypothesis generating in the service of furthering research aimed at uncovering potential mechanisms underlying the depression/insomnia comorbidity. Analyses of sleep data in extant datasets of reward processing impairments in depression as well as original projects aimed at exploring potential sleep, circadian rhythm, and reward interactions in depression are encouraged.