That startling jolt or falling sensation right as you're drifting off to sleep is a common experience shared by many. It's known as a hypnic jerk, and while unsettling, it's harmless and widespread.
Medically termed a hypnagogic jerk or sleep start, this occurs during the wake-to-sleep transition. Scientists aren't fully certain of the cause, but the leading theory points to a brief mismatch in brain signals: the conscious, wakeful part of your brain begins to shut down while muscles haven't fully relaxed, prompting a reflexive twitch as if to catch a fall.
