Sensation of falling, and then "jerking" awake just as one drifts to sleep. It is a normal sensation experienced by many people.

Hypnic jerk is the sensation of falling and then a physical jerk into wakefulness, usually during Stage 1 sleep. It is also called as sleep starts

A hypnic or hypnagogic jerk is an involuntary muscle twitch (more generally known as myoclonus or a myoclonic twitch) which often occurs during the transition from wakefulness to sleep (see hypnagogia). It is often described as an electric shock or falling sensation, and can cause movement of the body in bed. Hypnic jerks are completely normal, and are experienced by most people, especially when over-tired or sleeping uncomfortably.