Hi there,
My name is Nesim and I am gonna try and explain as well as describe what does it mean to over-sleep.
Before we get into it you must know that all of the religions even though there is only 1 real one are teaching us not to over-sleep. Now, your body basically gets a shut down. Your organs aren't working (most don't).
And your body prefers a 6-8 hours sleep. For me it's a 6 hours sleep. Because if I sleep more then 8 hours I fell tired and can't get through my day easily. Same if I am not sleeping at least hours, then I am sick I feel very very tired and I could even throw up.
So I think yes, your heart rate goes below minimum when your asleep and so perhaps over-sleeping can possibly bring more chance for a stroke.
Maybe that not a scientifically proven one, but I think you should be over-doing anything in life which is same for the sleep.
Best way is usually 6 hours and If you feel very tired then 7. Not even 8 hours is good (for me at least).
Hope this helps you out.
Best Regards,
Nesim