I've been thinking about this..... and the few times I've had SP happen, which has been around 10 occurrences, over the 20 some years since it first happened......
It really seems to me that every time it'd happen and I'd become aware & alarmed, the episode would last longer and become very unsettling.
However, the last few times I had SP, I knew what was happening.... I actually tried to embrace the paralysis, to turn it around and possibly even fall back asleep. Except when I did that, the episode would seem to end very quickly, and I'd end up wide awake fast....
About a year ago was my last SP experience, and it was brief indeed. Actually....... It rather upset me more that it didnt last longer, and I couldn't go back to sleep. It was on a day that it wouldn't have hurt to have slept in on..... 8)
The only comparable feeling I know of, especially when first becoming lucid and having the "presence", being watched, or being restrained sensations when going into SP..... is when I've taken a serious dose of DMT. It wouldn't surprise me at all, if there's some odd biological connection between SP, and the effects we get off of DMT and/or curare-type drugs.....