Why Your Body Jerks Right Before You Fall Asleep — A Biologist Explains The ‘Hypnic Jerk’

Hypnic jerks are common, usually harmless muscle twitches that happen as the brain shifts from wakefulness into sleep.

The leading explanation is a normal sleep-onset process, when falling muscle tone lets a brief motor signal slip through.

A popular evolutionary idea says the twitch may have helped tree-dwelling ancestors avoid falling, but direct evidence is lacking.

The piece helps separate a well-supported biological explanation from a plausible but unproven origin story.