Background Parkinson's Disease (PD) involves a complex interplay of non-motor symptoms—sleep disturbances, depression, and apathy—that significantly impair quality of life. However, the moderating role of depression in the sleep-apathy relationship remains poorly understood. Objectives This study investigated whether depressive symptoms moderate the link between sleep disturbances and apathy in patients with PD. Methods Two independent cross-sectional studies were conducted (Study 1: 197 PD, 394 controls; Study 2: 100 PD, 178 controls). Sleep quality was assessed via the Pittsburgh Sleep Quality Index. Depression and apathy were measured using the Neuropsychiatric Inventory (Study 1) and the Geriatric Depression/Apathy Scales (Study 2). Moderation analyses were performed across cohorts. Results Depressive symptoms were associated with moderation patterns in the association between sleep disturbance and apathy in PD patients, but not in controls. Specifically, sleep latency interacted with depressive symptoms to predict apathy severity (Study 1), and in Study 2, this interaction was particularly evident for “dropped activities/interests,” which influenced cognitive apathy. This pattern remained directionally consistent across sensitivity analyses that adjusted for anxiety, fatigue, pain, and motor severity. Conclusions Depressive symptoms may be involved in interaction patterns between sleep disturbance and apathy in PD, although these findings were attenuated after adjustment for broader symptom burden. These findings underscore the need for integrated, symptom-focused management strategies that address interactions among non-motor symptoms in PD.
Feng et al. (Fri,) studied this question.