Where are we supposed to put our arms while we sleep?

I’m (almost) forty years old and I still spend half of every night trying to figure this out.

Crossed on the chest feels too corpse-like. At the sides ok but neither palms up nor down is comfortable.

When I’m on my side, forget about it. No idea in hell where these damned things are supposed to go.

On the stomach they jut awkwardly at the sides due to low shoulder mobility. I mean there is really no solution to this serious problem.