Background and purpose Many patients with Parkinson's disease (PD) experience sleep-related symptoms. Studies in other populations indicate that melatonin can increase sleep efficiency, decrease nighttime activity, and shorten sleep latency, but there has been little research on the use of melatonin in PD. The purpose of this study was to compare the effects of two doses of melatonin to placebo on sleep, daytime sleepiness, and level of function in patients with PD who complained of sleep disturbances. Patients and methods A multi-site double-blind placebo-controlled cross-over trial was employed; 40 subjects completed the 10-week protocol. There was a 2-week screening period, 2-week treatment periods, and 1-week washouts between treatments. Nocturnal sleep was assessed by actigraphy and diaries, whereas daytime sleepiness and function were assessed by the Epworth Sleepiness Scale (ESS), Stanford Sleepiness Scale (SSS), and General Sleep Disturbance Scale (GSDS). Results Repeated measures analysis of variance revealed a significant improvement in total nighttime sleep time during the 50mg melatonin treatment compared to placebo. There was significant improvement in subjective sleep disturbance, sleep quantity, and daytime sleepiness during the 5mg melatonin treatment compared to placebo as assessed by the GSDS. Conclusions Although we found a statistically significant improvement in actigraphically measured total sleep time on 50mg melatonin compared to 5mg or placebo, this small improvement (10min) may not be clinically significant. However, the significant improvement found in subjective sleep disturbance suggests that these modest effects may be clinically relevant in this patient population.