Out of eight studies with 409 PD patients, seven studies entered a meta-analysis that showed melatonin improved sleep quality and insomnia, as indicated by Pittsburgh Sleep Quality Index (PSQI) (MD=−1.75 [−2.94 to −0.55]; p=0.004) and Epworth Sleepiness Scale (ESS) (MD=−1.07 [−1.87 to −0.27]; p=0.009). Melatonin showed no significant impact on objective sleep parameters. However, sleep onset latency (MD=−9.74 [−17.47 to −2.02]; p=0.001) and total sleep time (SMD=0.84 [0.47 to 1.21]; p<0.00001) improved, favoring melatonin, after adjusting for heterogeneity. Melatonin did not significantly change REM Sleep Behavior Disorder Screening Questionnaire (RBDSQ) (MD=0.71 [−0.44 to 1.86]; p=0.23). No significant differences was found between melatonin and clonazepam in ESS (MD=−2.65 [−5.36 to 0.06]; p=0.06).

Melatonin is a well-tolerated treatment that improves sleep quality, sleep onset latency, and total sleep time in PD. However, improvements in subjective sleep quality assessments did not meet minimal clinically important difference, which limits the clinical significance of these findings. Additional research is required to determine whether these benefits translate to other objective sleep parameters.