The sleep-wake cycle in Parkinson's Disease (PD) is profoundly disrupted, but less is known about circadian rhythm in PD and its relationship to other important clinical features. This study compared rest-activity rhythms in healthy older adults and PD patients with and without hallucinations. Twenty-nine older adults and 50 PD patients (27 with hallucinations, 23 without) were assessed using wrist-worn actigraphy for 5 days. Disease-related and cognitive data were also collected. PD patients demonstrated reduced amplitude of activity (F = 12.719, P < 0.01) and increased intradaily variability (F = 22.005, P < 0.001), compared to healthy older adults, independently of age, and cognitive status. Hallucinators showed lower interdaily stability (F = 7.493, P < 0.01) significantly greater activity during “night-time” (F = 6.080, P < 0.05) and significantly reduced relative amplitude of activity (F = 5.804, P < 0.05) compared to nonhallucinators, independently of clinical factors including motor fluctuations. PD patients with hallucinations display altered rest-activity rhythm characterized by an unpredictable circadian pattern across days, likely arising from damage to brainstem and hypothalamic sleep centers. Treatment of sleep and rest-activity rhythm disturbance is an important target in Parkinson's Disease. © 2008 Movement Disorder Society.