As I read before, you’re in a SP while sleeping, this makes your body paralysed, you cannot move.
Everyone changes positions while sleeping. How come? I thought we were paralysed while sleeping.
“Unwilling movements”, doesn’t that mean to stay steady, like to NOT FALL OFF THE BED?
I think SP is only active during REM sleep. That leaves plenty of opportunity for you to move around a bit during NREM sleep. 
Yes, we are only paralyzed in REM sleep, during the regular 4 stages of sleep we are free to move about, that explains why sleepwalking is possible.