All of my three often sleep on their backs, usually in a funny twisted way....don't know why.
I always wonder if it's partly because their little legs are so light, so easy to hold up in the air, lol. My Sheltie never sleeps on her back (only on her side). I love the Corgi sleeping positions! Darcy keeps me incredibly entertained, even when she's sleeping.
My three corgis love to wedge themselves at the point where the floor meets the wall. Tasha will also wrap herself around the ceramic base of the toilet, on the tile floor, with all fours up in the air. It has to take some talent to find this comfortable!!!
All the best from Williamsburg: Nan, Bear, Tasha and Linus