This is not strictly to do with AF but I know a lot of people on this board suffer from lack of sleep and may find this reassuring.
This research indicates that sleeping more than 6.5 hours is associated with cognitive decline, with a sweet spot of 4.5 to 6.5 hours: theconversation.com/sleepin...
I wouldn't say this is definitive, this is just one study among many, most of which do indicate a longer sweet spot. It would appear that it is the quality of the sleep that matters and not so much its length.
However, as I typically sleep 5.5-6 hours a night, I'll take this at face value!
Mark