Sleep positions

One thing I envy about 4 legged-animals (aside from how walking on four legs instead of two seems overall much more comfortable. No back strain. Walking upright evolved fairly recently and our backs still haven't totally adapted.) is the variety of positions they can comfortably sleep in. I can pretty much sleep comfortably on my side or to a lesser degree on my back. But my cat and dog on the other hand! They sleep on their backs and stomachs and every variation of laying on one's side and curled up in a ball and stretched out long and in a weird u-shape, with their heads up or down or to the side, with their legs under them or beside them and every possible variation of these positions. The one I most envy is being curled up in a ball. Or in other terms, the cat croissant. It seems very comfortable.