"unintentional napping"

I used to 'unintentionally nap' all the time since I was a teen.

Then I discovered I had sleep apnea. No more napping. And dementia has been linked to sleep apnea.

I wonder if this study is missing some quite big issues here.

> I wonder if this study is missing some quite big issues here.

Definitely seems like a possible "correlation is not causation".

I would find it hard to believe that sleep is somehow biologically harmful.

So, like your example, excessive or unintentionally napping is instead a sign of an underlying issue or poor health. Possibly the body trying to sleep to repair some damage?