You may never have used or even heard the term ‘hypnic jerk,’ but chances are you’ve definitely experienced one.
How many times have you felt this? You’re lying on the couch or in your recliner. You’re just about to drift off to sleep. Then, you experience a feeling of falling, which jars you awake without warning. You’ve just experienced a hypnic jerk.
No, I’d never heard the term, either.
But I definitely felt the phenomenon. They’re unpleasant and terrifying.
Sometimes, when I write about a new word being added to the dictionary, it’s an annoying fad of a word we hope will soon go away. But this time, the word (or more appropriately, the phrase) is a real word that’s been used for almost 50 years.
The Oxford English Dictionary recently added the term in its latest update. It defines it this way:
A sudden involuntary spasmodic or convulsive movement of the body or a part of the body, occurring when a person is beginning to fall asleep.
The first few times I felt it, I thought it must be something only I experienced. When you don’t have a name to call it, it’s hard to even describe it to someone else.
The OED suggests the word came into usage in about 1975. How did we miss it that long?
The Sleep Foundation tells us that hypnic is short for hypnagogic, which describes the transition from wakefulness to sleep. It also says they “typically only affect one side of the body, such as your left arm and left leg.”
That surprised me a little; I’ve never noticed it affecting only one side. The ones I’ve experienced seem to most affect the torso, so it feels like the whole body takes part in it rather than either side.
So what causes a hypnic jerk? Physiologically, researchers think it could be a “misfire” in the part of the brain that handles the “startle” response. The Sleep Foundation website lists four factors that may make the phenomenon more common.
But I wouldn’t recommend trying them just to experience it for yourself…if you’re lucky enough to never have felt one before!