"Myoclonic jerk," said the first sleep researcher I reached on the phone.

"I beg your pardon!"

"Not you," he said. "That twitch."

But when I told him it only happens once, as I'm falling asleep, he changed his tune.

"Oh, that's just a hypnic jerk." He gave me the name of another researcher. As jerks go, hypnic ones aren't annoying enough to get much attention. The second researcher gave me the name of a third researcher. The third researcher, Dr. Mark Mahowald, director of a sleep disorder center in Minneapolis, wasn't exactly fascinated with the subject, either.

"As far as I know, we don't have a clue why that happens," he said casually. "I don't think we could come up with two reasons."

I like the second theory better, since it's more specific than the first: This theory says that, as you slide toward sleep, there's a point at which your muscles really let go. Your brain, which after all did evolve from a reptile brain, interprets this rush of relaxation data as a sure sign that you're falling down. And it tells your arms and legs to thrash around and keep you upright -- which, of course, you're not. So your misguided body slugs your sweetheart in the solar plexus.

This explanation dovetails with the mental experience that accompanies many people's hypnic jerks -- the thrash is often accompanied by quick little dreams of falling. They're not exactly dreams, says Mahowald, although scholars are increasingly questioning the definition of dreaming.

But what -- or who -- is a myoclonic jerk? The term, I discovered, is outdated. Now this disorder is known as Periodic Limb Movement, which isn't nearly as colorful nor as fun to say.

The disorder itself is like hypnic jerking gone loco. As the myoclonic jerker sleeps, his legs jump and twitch at terrifically precise intervals -- every 30 seconds, for instance.

"It's like some sort of metronome," Mahowald says. "You can extrapolate five minutes out, and you'll be right on." The twitching may last two hours, then fade away. Although the jerker tends to sleep right through all the fun, the "bed partner" may not. And so researchers pay more attention to this disorder.

Hypnic jerks, on the other hand, are "of no medical significance," according to Mahowald. "It's completely normal. You see people falling asleep on a bus or at an airport, and they'll often wake up with a little start. You see it not uncommonly in college libraries."