I'm trying to figure out why my back hurts when I sleep and I can't figure out the physics behind it. So I sleep on an air mattress. The mattress fits firmly into the frame, which puts a constraint on the mattress to where it cannot expand on it's sides. Think of the mattress as being in a box where the top part( the part you lay on) is the only part exposed. What I'm thinking is that since the mattress cannot expand outwards, the volume displaced due to me laying on it is pushing the lighter parts of my body up, such as my head, stomach and feet, which causes my spine to be misaligned. I am not sure if this is correct, because it seems that the only thing that would change is that there is more pressure inside of the air mattress due to it not being able to expand. I do, however, know for a fact that my mattress feels different when compared to the exact same mattress that does not have the same constraints that mine has.