It's a reasonably well established principle of psychology (real science, not pop-sci) that a surprising amount of what you perceive to be your emotional state is mediated through your body. That is, you experience a stressor, so the "low level" portions of your brain activate bodily reactions to that stress, and what your high-level brain perceives as stress is actually the bodily reaction rather than the original stressor. Exerting such control as you can on your body (since it is non-zero, but not total either) is a legitimate way to control your (perceived) emotional state, which then feeds back into the entire system.
(This also factors in to how hard it is for some people to figure out why they are stressed; the part of the brain trying to work that out isn't necessarily as connected to the stressor as you might intuitively think.)
I've been having some low-level morning sleep paralysis lately (it has come and gone my entire life, & it has never been remotely as bad as I've heard some people describe); recently I've discovered an easy way out of it is to just hold my breath (or really, just stop inhaling), which triggers just enough stress to break through the paralysis. YMMV.