Is it possible for your posture to "deteriorate" during sleep?

I'm actively working at fixing my posture (stretches, exercises, even a posture "brace/harness"), but all my progress seems to reset upon waking up in the morning.

No matter how loose and flexible I feel or how happy I am with my posture, after waking up in the morning everything is completely stiff again and the progress starts over. Literally everything is stiff and messes with my posture: neck, shoulders, back, hips.

I don't think it is my bed since it's only a few years old, but it could be. I'm also a side sleeper so I don't know if that changes anything. I also always notice I kinda "tuck" my head in/forward when on my side and it feels the most comfortable for sleeping, but probably doesn't help with my desire to rid myself of the dreaded nerd neck.

Does anyone have any opinions or advice?