Med thumb guy falling

It’s a sensation that most people recognize: You’re drifting off to sleep, your body starts twitching and you feel like you’re falling. Then, you’re out cold.

This involuntary muscle movement is called a hypnic jerk, and it’s as harmless as it is common. These spasms happen during the transition between wakefulness and sleep. Some people get them multiple times in a night, typically at the onset of a Rapid Eye Movement cycle, or REM sleep.

The twitching comes with increased sweat production and a faster heartbeat. While uncontrollable spastic movements may seem symptomatic of a larger health problem, hypnic jerks are widely considered to be a meaningless quirk of being human.

In fact, they’re are such a non-issue that there’s virtually no research on them, according to Carl Bazil, a neurologist and the director of the Columbia University sleep center. That’s saying a lot, because there’s been research on everything. Bazil compared hypnic jerks to another universal and wholly benign involuntary reaction: yawning.

(But even yawning has been the focus of published studies.)

Does everyone get them?

We think so. But some more than others. According to Bazil, sleep deprivation and stimulants, including caffeine and prescription drugs, can increase the frequency and severity of spasms. Occasionally, patients who undergo sleep testing for other disordered behavior will inquire about their nighttime twitching, and Bazil reassures them that hypnic jerks are nothing to, um, lose sleep over.

In decades of conducting sleep studies, Bazil says only one patient has sought treatment specifically for hypnic jerks. In this lone case, the spasms occurred every time the patient fell asleep and were severe enough to startle him awake, eventually causing sleep deprivation.

Do hypnic jerks serve any biological purpose?

Maybe at some time in our evolutionary past, but not anymore. One theory credits hypnic jerks with protecting our primate ancestors who lived in trees. The involuntary muscle twitches may have startled pre-homo-sapiens enough so they shifted from a sitting position into a sturdier sleeping position before falling asleep for the night.

Assuming you’re not sleeping a tree, your own hypnic jerks are just harmless curiosities.