I just like to check in, from time to time. It doesn't usually paralyze people, though.
Edit: You were probably in the weird state between awake and sleeping. Sleep paralysis is something that happens naturally, when you're in deep sleep (people who toss and turn aren't sleeping deeply). If you start to wake up before it wears off, the panic of it can either cause a nightmare or general hallucination. Because you're in a state of panic, your brain will always produce something scary. It's almost always going to be shadow people.