Many forms of dementia destroy dopaminergic neurons, and this process begins much earlier than symptoms of dementia itself manifest. From what I’ve gathered from reading studies across the years, less dopaminergic capacity seems to be associated with less restful sleep.
The conclusions of this study seem to have it backwards. It’s not lack of sleep that causes dementia, it’s early dementia that causes insomnia.
Edit: it’s also possible that a third factor causes both of these. For example, both people low on dopamine and the sleep-deprived are likely to turn to coffee to keep them functional. If they drink too much coffee, they might accidentally poison themselves into dementia, because some of the Harmala alkaloids in coffee are neurotoxic and like to deposit themselves in brain fatty tissues for a long time.
The conclusions of this study seem to have it backwards. It’s not lack of sleep that causes dementia, it’s early dementia that causes insomnia.
Edit: it’s also possible that a third factor causes both of these. For example, both people low on dopamine and the sleep-deprived are likely to turn to coffee to keep them functional. If they drink too much coffee, they might accidentally poison themselves into dementia, because some of the Harmala alkaloids in coffee are neurotoxic and like to deposit themselves in brain fatty tissues for a long time.