What’s a hypnic jerk?

 Did this ever happen to you, your mind begins to wander off, fading into the nightly oblivion and you start drifting off to sleep. 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?

They are most common in children when our dreams are most simple, and they do not reflect what is happening in the dream world - if you dream of riding a bike you do not move your legs in circles. Instead, hypnic jerks seem to be a sign that the motor system can still exert some control over the body as sleep paralysis begins to take over. Rather than having a single “sleep-wake” switch in the brain for controlling our sleep (i.e., ON at night, OFF during the day), we have two opposing systems balanced against each other that go through a daily dance, where each must wrest control from the other.

Deep in the brain, below the cortex (the most evolved part of the human brain) lies one of them: a network of nerve cells called the reticular activating system. This is nestled among the parts of the brain that govern basic physiological processes, such as breathing. When the reticular activating system is in full force, we feel alert and restless - that is, we are awake.

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 are 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.

Researchers have come up with two hypotheses that may explain hypnic jerks.

Hypothesis 1: Your body twitches as daytime motor control is overridden by sleep paralysis. When you’re asleep, your body is paralyzed. This is called REM sleep atonia and it 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 down to your finger to make it move when you’re awake is no longer enough when you’re under REM atonia. However, sometimes, during this wrestling match between the two subsections of your brain, some motor neurons are fired randomly, causing your body to twitch.

Hypothesis 2: Another theory is evolutionary, stretching back to our primate ancestors. Frederick Coolidge, a psychologist at The University of Colorado, has suggested that a hypnic jerk could be "an archaic reflex to the brain's misinterpreting the muscle relaxation accompanying the onset of sleep as a signal that the sleeping primate is falling out of a tree. In this theory, it means that we’ve inherited some monkey brain routines that no longer serve any purpose. According to this hypothesis, one of them is a reflex that jolts you awake when you’re falling from a tree.

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 eyelids become heavy and your breathing slows down. 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! Oh no, crap, poop! Brace for impaaaact!”

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, which should be okay if you don’t wear steel-toe boots in bed.

Source:

BBC: Why your body jerks before you fall asleep.

McGill Office of Science and Society: What are Hypnic Jerks?

Business Insider: Why You Sometimes Feel Like You're Falling And Jerk Awake When Trying To Fall Asleep