Hypnic jerks (or: Proof I didn’t mean to fall asleep in class)

Has this ever happened to you? There you are, drifting softly into sleep, when all of a sudden –


– your arms flail, your legs shudder and you’re awake again. Oh, why DO we twitch as we fall asleep?

The myoclonic twitch (a brain-stem reflex also known as a hypnic jerk or hypnagogic startle) is experienced while sleeping or while falling asleep and is thought to be associated with the reticular activating system.

In plain English, as you fall asleep, your body goes through certain steps to relax and prepare for sleep. Your breathing slows, your muscles relax, and your brain shuts off certain sensory inputs – including those from your inner ear. It seems that if the brain is still too alert when it halts input from the inner ear, this triggers a reflex to startle and reach out – not unlike the sensation you have right after you realize you’re falling.

Chin up

We’ve all dozed off in class, waking up with a snap as our heads jerk back up. And it usually feels like the whole class (and the guy by the blackboard) has seen the dramatic movement.

Well, for those whose entire formal education was punctuated by frequent hypnic jerks, here’s something that might help you feel a little less embarrassed. It turns out that hypnic jerks happen most frequently to those who are resisting sleep – not those who are trying to succumb to it. So chin up, folks. You didn’t mean it.

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