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:…
Big idea: a common cause may be jaw development and the resulting lack of breathing (during the day and during sleep) in large parts of the population.
Jaw development has a strong claim to be the fundamental, common cause of poor breathing and sleep because it's derived from 2 fundamental differences between humans and animals: agriculture and speech.
It's a long chain of causation, and took me awhile to learn about, but it may be what you're looking for.
Of course it could also be the case that some people are just staring at screens too much before bed and getting poor sleep. But bad sleep is an epidemic and there is evidence of this all over the world, and going back hundreds of years. The books I reference tell of orthodontists from the 1800's who noticed this problem, etc. Our diets drastically changed, which caused poor jaw development, which causes bad breathing, which causes bad sleep (I learned about this from my dentist).
There are many factors that restrict airways, including weight. Remember that the average American male is about 30 pounds heavier in 2020 than in 1960 (and similarly for females). It's hard to imagine that not having an effect of breathing and sleep (and thus dementia). But I think animals can be fat and still have plenty of air for breathing; the same isn't true for humans.
Previous comment I wrote on this including recent books (2018-2020): https://news.ycombinator.com/item?id=23435964
That article is "fitful nightly sleep linked to chronic inflammation and hardened arteries". That is another correlation, not a causation. So I would suggest that bad sleep, inflammation, hardened arteries, and dementia have a common cause in jaw and airway development. This is a lot harder to see because it took place over 10,000 years and it takes 30-50 years to kill you. You can have bad breathing your whole life but still be functional. It's very related to tooth grinding/bruxism, which is an epidemic as well.
Other threads on Hacker News: https://news.ycombinator.com/item?id=24247066