The whole deficit thing is interesting. I've felt that I do that too, but I've also heard people say that sleep doesn't work that way. It definitely could just be that any time I'm sleeping more than 8 hours I'm just being lazy.
As I wrote in my initial comment the occasional disturbed night is not a big problem when I'm well rested. Under those circumstances I never feel the need to get extra sleep to compensate for the lost hours.