You may have never heard of a “hypnic jerk,” but the odds are you’ve experienced one.
A “hypnic jerk” is not a really annoying wannabe Reveen: rather, it’s one of those involuntary muscle movements you get just when you’re falling asleep. Often (at least in my case), they’re accompanied by a dream that involves falling, which means you feel like your falling as you fall asleep with the ironic result that you fall awake, instead.
At this point in a column, I’ll often write something like, “a recent study has shed new light on this behavior,” but the fact is, it hasn’t. Relatively little research has been done on “hypnic jerks,” “sleep starts,” or, if you want to sound really erudite, “hypnagogic myoclonus” (myoclonus is the scientific name for any sort of involuntary muscle spasm, and hypnagogic refers to sleep).
That doesn’t mean there aren’t theories. A long-ago study was able to induce hypnic jerks in subjects by having them push a button whenever they heard a low tone. Since the subjects were sleep-deprived college students, it wasn’t too long before they nodded off…and often the next sounding of the tone would cause them to twitch, after a lag of a few seconds.
The conjecture was that the subjects knew consciously they were supposed to stay awake. The tone jolted their semiconscious brains, which then tried to scramble back to wakefulness again, producing the jerks.
This certainly fits well with my experience, since I’ve often jerked awake after dozing off during a soporific symphony or sermon.
On the other hand, I’ve also jerked awake after dozing off comfortably curled in my bed and wanting to go to sleep, so one size definitely doesn’t fit all when it comes to this phenomenon. And, yes, there’s another theory that the hypnic jerk may be the result of muscles relaxing into sleep, a relaxation the confused brain misinterprets as a sign of falling.
Oh, and you’re apparently more likely to experience this onset-of-sleep jerk when you’re sleeping uncomfortably or are overtired.
There’s another kind of hypnic jerk that takes place after we’re asleep, typified by twitching fingers or limbs. It’s this kind of twitching that we see in sleeping pets. “Oh, the cat’s chasing mice in her dreams,” we’ll say, looking fondly at a feline friend with flicking feet.
I don’t have a cat now, but I have had one in the past. And if you’ve never seen this phenomenon yourself, I direct you to the YouTube video embedded below of a mother cat hugging a twitching kitten, which will provide you with several dozen times the recommended daily dose of cuteness.
That video has become so popular (more than 18 million views) that no less august a site than National Geographic online decided to ask Dr. Nicholas Dodman, director of the animal behavior clinic at Tufts Cummings School of Veterinary Medicine, about it.
First, is the twitching kitten actually having a dream? Dr. Dodman says yes: dogs and cats, just like people, go through periods of rapid-eye-movement (REM) sleep, when the brain is very active. That’s when we dream, so there’s no reason to believe that animals don’t dream, as well.
Second, why the twitching? Dr. Dodman explains although large “anti-gravity” muscles like those that move your legs are shut off when we sleep (because they are activated by a neurochemical called serotonin, which the brain shuts down during sleep), muscles used for precision movements—fingers and toes for us, paws and whiskers for cats—remain active, and so may respond to signals from the brain during REM sleep.
While we’re aware of the jerks that occur when we’re falling asleep, jerks and twitches in the middle of the night may only be noticed by bed partners. (There’s a joke in there somewhere about people who have jerks for bed partners, but I don’t think I’ll try to tease it out…)
In most people, hypnic jerks occur just once or twice a night, but people with a disorder called periodic limb movement have their sleep disrupted by muscle twitches that occur every thirty seconds or more.
Hypnic jerks affect around 10 percent of the population on a nightly basis, and almost 80 percent of people are affected occasionally, so it’s really nothing to worry about.
Besides, who knows? Capture your nocturnal twitching on video, and you, too, could be a YouTube sensation.
Provided, of course, you’re as cute as a kitten.