Testing gentle sounds during sleep

Inclusion Criteria: * Signed informed consent * Diagnosis of polysomnography-confirmed isolated REM Sleep Behavior Disorder (iRBD) based on international criteria (ICSD-3), combined with EITHER * UPDRS III without action tremor ≥ 4 AND abnormal olfaction, OR * diagnosis of PD along international criteria for less than 2 years Further inclusion criteria are: * no dopaminergic treatment and no foreseen start of such treatment during duration of the study * ability to apply the intervention, alone or with help of a co-habitant, stable living situation * sufficient language skills in German, French or Italian * negative pregnancy test for women of child-bearing potential Exclusion Criteria: * Suspected or known non-compliance to other therapies * current or recent participation in another clinical trial * extended absences * hearing impairment that prevents hearing the tones for auditory stimulation * non-responder to auditory stimulation during screening * clinically significant concomitant disease or unstable condition * Apnea-Hypopnea-Index (AHI) \> 15/h or under Continuous Positive Airway Pressure (CPAP) treatment * Restless Legs Syndrome * meeting criteria for diagnosis of atypical Parkinson syndrome * diagnosis of Dementia or Montreal Cognitive Assessment (MoCA) \< 24 * severe Depression or other psychiatric disorder * regular use of benzodiazepines and other central nervous system depressant substances * current or recent history within the last year of substance abuse disorders or chronic alcohol consumption * recent or planned major surgery * history of allergies and hypersensitivity relevant for electrode application or medication allergies * additional exclusion criteria for PET imaging * any criterion that may pose the participant at risk * breastfeeding, intention to become pregnant, or unwillingness to use medically reliable contraception for women of child-bearing potential