Finally! An answer!
It's a lazy Sunday afternoon. You've actually done your chores for the day and you're ready to relax. You find a nice, warm spot on the couch where the sun hits it just right, and you've started drifting off into a gentle nap.
Suddenly, your body jerks — hard. You're jolted awake, and maybe you're sweating or your pulse is racing.
You might even have had a dream that you were falling from a cliff and pulled yourself back to wakefulness so quickly you end up in a pile on the floor. Your animals may be judging you silently from the very same couch you were just sleeping on. Ouch.
What you've just experienced is called a hypnic jerk, and though scientists don't really know what causes it, they've surmised a couple of things about it. A hypnic jerk — or “sleep start” — is actually a natural reaction from your body when you're falling asleep too quickly.
There might be other factors involved, such as stress, alcohol or caffeine. This might stop you from relaxing properly, keeping your brain awake and convincing itself that your lowering heart rate is actually because you're in danger — not because you’re falling asleep.
While a hypnic jerk isn't a symptom of anything being wrong, it can be a scary experience. A portion of some sleep starts include "a rapid heartbeat, quickened breathing, sweat, and sometimes "a peculiar sensory feeling of 'shock' or 'falling into the void,'" according to Marianne Middleton of Lawrence Memorial Hospital Sleep Disorders Center.
So while it’s not dangerous to your health, it can cause you to develop a nasty "vicious cycle" in which you lose sleep because you develop anxiety over falling asleep. Which means your body tricks you into being afraid of falling asleep because it jerks itself awake in what might be a terrifying experience.
Not cool, brain!
The good news is that hypnic jerks mostly affect people who have inconsistent sleep schedules. One way to try and eliminate the possibility of experiencing a sleep start is train yourself to go to bed on a proper schedule, and make sure not to take in caffeine or alcohol before you go to sleep.
Hypnic jerks can be well... jerks, but you don’t have to let them ruin your sleep.