
That sudden sensation of falling followed by a body jerk is called a hypnic jerk or sleep start. It usually happens during the transition from wakefulness into sleep. A leg, arm, or even the entire body may suddenly twitch, sometimes accompanied by a vivid sensation of tripping or falling.
These movements are common and generally harmless. Stress, sleep deprivation, and stimulants may make them more noticeable. The body can briefly surprise itself on the way to sleep.
© Sleep Foundation