**1.6 The need for a systematic literature review and meta-analysis**

The unanswered questions described above indicate a clear need for a more precise portrait of sleep disturbances in PD. A systematic review of the existing literature is warranted. Some authors have undertaken the effort in recent decades [38–41], but contradictory results from objective data and variations in the reporting of subjective data resulted in questions remaining about sleep alterations in the population. Lack of uniformity in NPA prevalence measurement and reporting yielded significant variability in prevalence rate estimates. Meta-analytic methodology involves pooling all samples into one group to conduct a quantitative analysis of data from previous research, with greater statistical power than when analyzing each study's data alone. This method would likely allow for a clearer interpretation of the current literature on the subject.

The present study was designed to systematically review the existing literature on sleep disturbances in PD. More precisely, the objectives were (1) to compare sleep in patients with PD to sleep in healthy controls and (2) to assess the prevalence rate of NPA in patients with PD. We hypothesized that the sleep of patients with PD would be significantly different from that of healthy controls.
