Heavy sleep that it falls off the surface?

We don't have cats in our house. But now in my campus we have a lots of cats wandering around. There are a few occasions that I saw the cats sleeping at the edge of verandah. But they seems to be able to keep a good balance that they would never fall from there. Perhaps I wasn't at the luck to see them fall down from the edge. It could be funny to see that though.

My cats do this all the time, they will dose off nad fall. One time I watched my cat slowly close her eyes, and roll off her cllimber in the living room.

Yes! Indeed I was astonished! They are supposed to have very good balance!