What's Happening When Your Sleepy Body Starts to Jerk
By: David B. Clear
Tags: Neuroscience, Sleep, Health, Biology, Science

Why Your Body Sometimes Jerks As You Fall Asleep

A closer look at hypnic jerks

Images by the author (CC BY-SA 4.0)

Ahh… sleep. How nice. You turn off the lights. You close your weary eyes. You sigh. You relax. Your breathing slows down. Your mind begins to wander off, fading into the nightly oblivion.

Then…

You stumble, trip, fall. Your body jolts. Your leg kicks. Your heart pounds. Huh? What happened? Did you mistakenly fall asleep on a trapdoor?

Nope. You simply experienced a hypnic jerk.

What’s a hypnic jerk?

A hypnic jerk, or sleep start, is a phenomenon that occurs when your body transitions from wakefulness to sleep. It involves a sudden involuntary muscle twitch and is frequently accompanied by a falling or tripping sensation. It’s that strange muscle spasm that happens when you’re lying in bed, trying to sleep, and are suddenly jolted awake because you feel like you stumbled over something.

Hypnic jerks are common and benign.

But what causes them? Well, no one really knows. It’s still a mystery. However, researchers have come up with several hypotheses that may explain them, with the following two being the most popular.

Hypothesis 1: Your body twitches as daytime motor control is overridden by sleep paralysis

How is it that a bedfellow of yours doesn’t wake up pummeled and bruised if you have a dream about a boxing match? Is it because they’re having a complementary dream where they’re blocking all your jabs, hooks, and other punches?

Nope. The person sharing the bed with you doesn’t get pummeled because when you’re asleep, your body is paralyzed. This is due to something called REM sleep atonia, which prevents you from acting out your dreams.

REM atonia works by inhibiting your motor neurons. It does so by raising the bar on the amount of electricity the brain must send down a motor neuron to trigger a movement. So, for instance, the little bit of electricity that your brain sends to your finger to make it move when you’re awake is no longer enough when you’re under REM atonia.

When you’re asleep, your body is paralyzed. This is due to something called REM sleep atonia, which prevents you from acting out your dreams.

Now, the thing is that there is no single on/off switch in your body that inhibits all your motor neurons at once. Instead, the subsystems of your brain that handle sleep need to wrestle control from the subsystems that handle wakefulness. And sometimes, during this wrestling match, some motor neurons are fired randomly, causing your body to twitch.

Hypothesis 2: Your brain thinks you’re a monkey falling off a tree

Image modified by the author. Illustration source: Alessandro D’Antonio/Unsplash

Imagine you’re a monkey and the last rays of sunlight have just disappeared behind the green forest canopy. It’s getting dark, and you say to yourself: time for sleep. Your brain begins to ooze some melatonin into your bloodstream and you yawn. Drowsy, you settle down on a comfortable tree branch.

Your eyelids become heavy and your breathing slows. The outside world begins to fade. Sounds become distant.

At this point, the subconscious part of your brain takes over. “Perfect,” it says, “time to boot up the dream images.” Your brain initiates the dream procedure, and just when you’re about to nod off completely, it notices that all your muscles have suddenly and unexpectedly relaxed. “Holy Banana!” your brain screams panic-stricken, “Mayday! Mayday! We’re in freefall! Dammit! Wake up! Wake up! Shit, crap! Brace for impaaaact!”

As you’re probably aware, we humans descend from primates who lived and slept on trees. This means that we’ve inherited some monkey brain routines that no longer serve any purpose. Among them, according to the monkey-fall hypothesis, is a reflex that jolts you awake when you’re falling from a tree.

You see, when a monkey is unexpectedly soaring through the air, its muscles no longer have to prop it up and so they go limp. Confusingly, however, your muscles also go limp when you’re sleeping.

So, when you drift off into sleep and your muscles relax a little too fast, your groggy brain sometimes misinterprets this for falling off a tree. As a result, your brain freaks out and triggers a reflex that startles you awake in an attempt to prepare for an imminent crash onto the forest floor. Little does your brain know, in its sleepy state—and that you no longer live in trees.

What’s clear either way

Hypnic jerks are involuntary muscle contractions that occur during the transition from wakefulness to sleep. They’re most likely to occur if you’ve been gulping down too much coffee, have been stressed or sleep-deprived, or did some vigorous exercise before going to bed. About 70% of people have experienced them. Even so, they are not well understood.

Either way, hypnic jerks are benign and nothing to worry about. The worst that can happen is probably an occasional kick against the shin of whoever is sharing the bed with you.