The purpose of this study is to assess the link between sleep and the clearance of waste products from the brain. This will be assessed in a group of healthy older adults.

First, the investigators want to assess the relationship between sleep parameters and markers for brain clearance, assessed with MRI and blood samples. Second, the investigators want to probe whether associations can be influenced by modulation of sleep parameters by means of auditory stimulation.

Inclusion Criteria:

• Participation in previous longitudinal cohort studies at the study site
• Ability to provide written and verbal informed consent
• Fluency in the German language
• Stable home situation that allows for reliable application of the intervention for the duration of the study
• Ability to apply the intervention for the duration of the study, either alone or with support from a co-habitant
• For women: Hormonal contraception, if menstrual cycle is still present or has been present within a year
• Good general health status, normal cognitive function

Exclusion Criteria:

• Failure or inability to give informed consent
• Inability to follow the procedures of the investigation
• For women: pregnancy or breastfeeding
• Presence of sleep disorders (e.g. sleep apnoea, narcolepsy, restless leg syndrome, insomnia, rapid eye movement (REM) sleep disorder)
• Shift-work or a home situation that requires several awakenings during the night (e.g. newborn)
• History of serious illness likely to interfere with study outcomes
• Presence or history of a diagnosed neuropsychiatric illness
• Current diagnosis of uncontrolled or poorly controlled diabetes
• Current cancer of the brain or unstable cancer undergoing treatment
• Evidence of serious atherosclerotic cardiovascular disease

Double (participant and outcomes assessor)