You’ve probably experienced it: just as you’re drifting off to sleep, your body suddenly jerks, and you’re jolted awake by a sharp sensation—like you’ve tripped off a cliff or missed a step. This unsettling feeling is surprisingly common, and scientists even have a name for it: the hypnic jerk.
Also called a sleep start, this sudden movement is a brief, involuntary muscle contraction that occurs as a person begins to fall asleep. Though usually harmless, it can be intense enough to wake you up—and leave you wondering what just happened.
So, what causes this strange and somewhat alarming phenomenon? As it turns out, the answer lies at the intersection of neuroscience, evolutionary biology, and sleep science.
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