Visual hallucinations (VHs) in Parkinson's disease (PD) can be a frequent and disturbing complication of the disease with 33% of PD patients undergoing long-term treatment experiencing VHs during the course of their illness. One line of evidence that is emerging as a possible risk factor in the occurrence of VHs is the sleep–wake cycle and sleep behavior in patients with PD. This study compared sleep patterns in a group of visually hallucinating Parkinson's patients with a group of nonhallucinating PD patients and an age-matched control group. Nocturnal sleep was assessed by actigraphy and diaries, while daytime sleepiness and function were assessed by a battery of self-rating sleep questionnaires. Compared with the control group both patient groups had more sleep-related problems and significantly altered sleep patterns, as measured by both actigraphy and sleep questionnaires. Patients who hallucinated however slept less than nonhallucinating patients and also had increased awakenings after sleep onset, reduced sleep efficiency, and increased daytime sleepiness. We propose that VHs in some PD patients may be a symptom of poor sleep and prolonged daytime sleepiness, suggesting that arousal may play a role in the genesis of the hallucination phenomenon.