Your comment prompted me to look for images of chimpanzees sleeping on google. A lot of them are putting their arms under their head just like we would because

Your comment prompted me to look for images of chimpanzees sleeping on google. A lot of them are putting their arms under their head just like we would because of their shoulders. Others find a root or rock to rest their head on (and they are also able to build temporary beds/nests in the tree at an incline). So I figure it'd be mostly the same for us.