Additional anecdote here, if I sleep 8 hours or more I feel horrible, the same as if I slept 12 (oversleeping) but I feel better than ever when I get 5-7.5 hours a night. I don't think there's a one size fits all, sort of like calorie counting
Are you able to "oversleep" even when you are not sick? I suspect that the few times that I've been able to sleep more than 9 hours have all been because of illness and/or bad quality sleep. I can easily stay in bed for 12 hours, but staying asleep is another matter.
Yup, though one might argue that I'm always sick, because I have mild allergies to just about everything airborne. Otherwise on a completely normal day, if I sleep for 8 hours I'll feel as if I overslept, especially if it's more than one night in a row