That sudden falling sensation you sometimes feel when drifting off to sleep is called a hypnic jerk (or sleep start). It’s totally normal and happens during the transition from wakefulness to sleep, usually in the first stage of non-REM sleep.

Here’s why it might happen: Muscle relaxation confusion: As your muscles relax, your brain might misinterpret the sensation as falling, so it triggers a reflex jerk to “catch” you. Evolutionary leftover: One theory says it’s an ancient reflex from our primate ancestors—if they started to fall asleep in a tree and lost balance,

the jerk would wake them and help them avoid falling. Stress, caffeine, or fatigue: These can increase how often or how intense the jerks feel. Brain misfire: As brain activity slows down for sleep, sometimes it sends out a stray signal to the muscles—like a little glitch.