Belly sleeps like this every night!! It used to worry me (his spine?!) but this is what he likes ...
He goes to bed first ...usually around 9 ish. I check on him later, when I go to bed, and he's always in this position. And of course I MUST rub his tummy. I get a little half growl/half groan from him. Like "get out of here!"
He has his own black leather chair that cradles him perfectly in this position.