Well I guess you're ready for an additional step.
You literally were paralyzed.
I've had this described as an individually-tailored hormone acting as a muscle feedback-loop block to the "cholinergic nervous system". I haven't researched this in some time, so that description sounds off. I think this is the entrance to the rabbit hole if you'd like to pursue science on it:
Terror (no, not fear.. Terror) is indeed common (probably guaranteed for first-timers). This happens, and is universal to (some?) humans enough to enter into mythology in a number of cultures (e.g. Newfoundland/Canada calls it "the hag" or "ag rog").
When awaking from sleep, parts of your brain are coming online at different times, and doing all kinds of fun things like interacting with one another in novel ways. Hence the vivid end-of-sleep dreams, and "inspiration" upon waking up. Well.. spoilers. Your post means you're beginning to notice the mechanics of your watchdog. Pulling the curtain back, as it were. Anyway..
If you'd like to experiment with the kind of terror probably stronger than substances and torture, then while in a good mood, already well-rested and on no substances: snack and nap. Don't meditate or the like, just rest until you sleep without having been tired beforehand.
The important thing I noticed is I have a strong heartbeat and fall asleep without being tired. This'll likely take under 15 minutes from beginning to end.
If you can sleep with your mouth open, please for the love of $deity pursue that. Yes you'll be fine when breathing through only one nostril, but.. that's not optimal.
Be alone, because the last thing you want is to have someone else (or a cat) be associated with the terror enough to need to face the touch of phobia of them you might get.
I've experienced it naturally, and have noticed that doing the above can trigger it. I've done so several times, and.. I've faced it. If you ever get there your posture will change.
I could give you additional tips on mindset and such, but I expect you'll glean that from this text or while you're in there.
For anyone else reading this -- for fuck's sake don't do this unless you already have some shaman in you. There are good reasons this stuff is nightmarish myth and not the ghost stories of religion.