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Is there any more to it than an automatic reaction before the "walking/talking" part of the brain has disconnected from the actual muscles? (Don't remember the names, but I do remember reading about it.)

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I think thats it, yes. ISTR that the brain 'unplugs' from the muscles as you enter sleep, to stop you acting out your dreams (though people who sleepwalk have problem with this). This "unplugging" can cause a sort of muscular spasm - quite a lot of people do odd "jerk"... as they go to sleep, apparently I do sometimes. And sometimes the spasm is enough to wake you up.

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That all sounds right to me.

I *always* spasm once when I'm about to fall asleep.  Mum used to be able to tell when I'd drifted off when I was a baby by the sudden jerk.  It's funny, cos I'm often conscious of it, and have to be careful what I think.  If I pay too much attention to the fact that I'm about to fall asleep, I wake up again.  I have to kinda notice, but ignore the spasm....

I usually only get the awake-with-a-start thing if I'm in a strange bed, which doesn't happen that often, more's the pity.

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IIRC that's right, and every now and then you can mentally wake up before you're "plugged in" again, although by the time you realize this / care, you jumpstart the process.  I've done that once or twice.  It's very, very strange, but doesn't last very long. Your body does an "oops" and turns on.

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Not always. It's possible to wake, become fully conscious and be utterly unable to move or speak. The first time it happened to me it lasted about a minute. There was an article on it earlier this year in either the Grauniad or the Observer and I thought, "Thanks. It would have been useful to have known that the time I almost DIED FROM TERROR."

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I was quite young when I learned about "sleep paralysis" and used to wonder whether the interesting experience would be worth the worry. Apparently it's often accompanied by feelings of dread, of some kind of malevolent presence, and of weird changes in one's body image (these feelings can be generated in a lab by passing changing magnetic fields through the brain). I like the theory that lots of cases of "alien abduction" are really strange experiences associated with sleep paralysis. Before aliens, people used to think that a witch had visited them or that the Devil was sitting on them when they had episodes of sleep paralysis.

Is it less terrifying when you know what it is and have had it before?

GCU Fun With Oscillating Fields