By Hannah Holmes
You are sinking into your pillow, the first flickers of dreamlike peculiarities playing across the inside of your eyelids. Then, finding yourself sliding down a hill or falling in a hole, you thrash so suddenly you wake up. As does your sweetheart, whom you've kicked out of bed.
"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."
There are, of course, theories, since theories rush in where angels fear to tread.
One is that hypnic jerks are a natural step in the body's transition from alertness to sleep. As you drift off, your body goes through some physiological changes to prepare for a few hours of restoration. Your breathing rate changes. Your temperature may change. And your muscle tone changes, too. Hypnic jerks, the theory goes, may just be a byproduct of that muscular transition.
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.
The traditional view is that true dreams only visit during REM (rapid eye-movement) sleep, later in the night. But a dopey, dozing-off brain gets its chuckles in the form of modest hallucinations or reveries. These pastimes are more closely related to daydreams than REM dreams. Anyway, Mahowald says about half the people in any given audience to which he speaks admit to the occasional hypnic jerks, and a dream of falling is a frequent companion to the twitch.
There are also more exotic, and more rare, versions of the sleepy-time surprise.
One is called an "auditory sleep start." Here, instead of waking with a twitch, you wake to a very loud snap or cracking sound that seems to originate in the center of your own head. Sounds very unpleasant. Another, a "visual sleep start," replaces the crack with a blinding flash of light, also coming from inside your noggin or your eyes. A final variation returns you to full consciousness with a "flowing sensation" that oozes over your skin. All the sleep starts may be accompanied by a grunt, as you exhale through sleepy vocal chords.
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."