2014 art, digitally colored version of this sketch.
Ridgebacks instinctively sleep on their stomachs. This is for a number of reasons. It's a good position should they be ambushed by a predator in their sleep, as they can easily scramble to their feet and get away. The hide and hair on their backs is thicker than it is on their bellies, which provides a bit of defense from predators and nuisances as well as acting as almost a sort of natural "blanket". It's also simply the most comfortable position for them, given their physiology. Their bellies have softer fur on them, which provides a natural cushion of sorts, and it's just more comfortable than laying on their backs due to the curvature of their spines.