The multimodal effect of circadian interventions in Parkinson's disease: A narrative review

Background: The circadian system and its dysfunction in persons with Parkinson's disease (PwP) has a clear impact on both motor and non-motor symptoms. Examples include circadian patterns in motor disability, with worsening of symptoms throughout the day, but also the existence of similar patterns in non-motor symptoms. Objective: In this narrative review, we discuss the role of the circadian system, we address the role of dopamine in this system, and we summarise the evidence that supports the use of circadian system treatments for motor and non-motor symptoms in PwP. Methods: A systematic search in PubMed and Web of Science database was performed and the final search was performed in November 2021. We included articles whose primary aim was to investigate the effect of melatonin, melatonin agonists, and light therapy in PwP.Results: In total 25 articles were retrieved. Of these, 12 were related to bright light therapy and 13 to melatonin or/and melatonin agonists. Most, but not all, studies showed that melatonin and melatonin agonists and light therapy induced improvements in measures of sleep, depression, motor function, and some also cognitive function and other non-motor symptoms. For some of these outcomes, including daytime sleepiness, depressive symptoms, and some motor symptoms, there is level 2 B evidence for the use of circadian treatments in PwP.Conclusions: Treatment with bright light therapy, exogenous melatonin and melatonin agonists seems to have not only positive effects on sleep quality and depression but also on motor function in PwP. Drawbacks in earlier work include the relatively small number of participants and the heterogeneity of outcome measures. Further large and well-designed trials are needed to address these shortcomings and to confirm or refute the possible merits of the circadian system as a treatment target in PwP.