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Why Do We Twitch When We're Falling Asleep?


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Channel: BrainStuff - HowStuffWorks
Categories: Biology   |   Health   |   Psychology   |   Science  
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A majority of people feel like they're falling when first going to sleep, resulting in twitches. What is this jerking night spasm?

This video was a collaboration with our friends at AllTime Numbers. Check out their video for 'Sleep in Numbers' here:


"Extreme hypnagogic jerk"

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Good evening BrainStuff. I'm Cristen Conger and we need to have a talk about your little sleep problem.

You know what I'm talking about... the one where you're just about to fall blissfully asleep, when you suddenly feel you’re falling, as if someone threw you out of a tree.

What's important is that you're not alone. According to the American Academy of Sleep Medicine, up to 70 percent of us experience this falling sensation and then twitch or jerk in our sleep.

Science and medicine refer to any involuntary jerking of our muscles like this as "myoclonus." I know it sounds like a cross between those weird bat things in The Empire Strikes Back and that movie about the haunted mirror, but there are all kinds of twitches classified as myoclonus.

Both contractions and relaxations of muscles fall under this category and the simplest example is when we hiccup. Most forms of myoclonus are fairly normal and are caused by a disturbance in your central nervous system.

The specific type of myoclonus that causes you to twitch in your sleep is called a "sleep start" or a "hypnagogic jerk." These are not hypnotists you've had a bad breakup with. No, hypnagogic jerks -- or "hypnic jerks" as the cool kids are saying -- happen just as you're transitioning between wakefulness and sleep. They're quite common, perfectly normal and some people sleep right through the twitching without even knowing it.

The brief jerking around of your limbs lasts only half a second or less. And one thing we know for sure is that they are not the same as the involuntary twitches you sometimes have during REM sleep while dreaming, or other movements caused by snoring or grinding teeth.

There are two theories as to what’s going on with hypnagogic jerks, but fair warning, neither have evidence to really back them up. And unsurprisingly, both involve our brains inferring that we’re falling… when we’re not.

The first is that hypnagogic jerks are part of our natural transition into sleep. It’s possible that our nerves misfire when our muscles calm down, because our brain assumes that their relaxation means we’re falling.

The second idea is that these spasms are reflexes we still retain as evolved primates. You know, if you believe in that sort of thing. This could be designed to keep you from falling out of a tree you’re sleeping in, causing you to quickly react. But as the old Japanese proverb goes “Saru mo ki kara ochiru.” Even monkeys fall from trees.

So myoclonus isn’t 100% understood. And we don’t know why hypnagogic jerks happen. But… there are a few things we believe can make them worse. Stress, too much caffeine, intense exercise, and smoking are all on the list. Y’know, the usual. But if you’re experiencing them constantly and they keep you from sleeping, you should go talk to your doctor.


while you were sleeping. Good Health (Australia Edition), Dec2009

A Case of the Jerks by Kaitlyn Syring, University Daily Kansan, February 28, 2008

ASK TIP SHEET. By: Romano, Andrew, Newsweek, 00289604, 3/27/2006, Vol. 147, Issue 13

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