I don’t know how I managed to get a sore back just from sleeping on it but I did exactly that on Saturday night. I woke up on Sunday morning and my lower right side was just aching. It wasn’t a sharp pain but an annoying and persistent ache like I had pulled something.
It definitely got better once I got up but that whole area felt tight all day. I tried to stretch it out and took a nice warm shower to get the muscles there all relaxed. I think that was enough for me to get through the day without any problem.
I have a friend who is a few years younger than I am and he says that he just wakes up sometimes and some random part of his body will just be sore, like he pulled a muscle in that area. This is even after not doing anything physically strenuous for weeks before. Getting old sucks.