I have this on a purely physical level too. I daren't actually relax my muscles, because if I do, shit dislocates. I'm constantly holding my body into the shape of a human, just like I'm constantly holding my mind into pretending to be closer to neurotypical than I am.
McNutcase
54) You have no idea what it would be like for your brain to be quiet or empty.
Nome
Since then, when folks say they're not thinking of anything, I've asked them what they mean.
And apparently, many of them literally mean they aren't actively having any thoughts. They may just be having a sensory experience, or calm and quiet.