Have you ever fallen asleep, only to suddenly wake up with a jolt? It's actually called a hypnic jerk.
The startling spasm is often described as a falling sensation or an electric shock. It tends to happen more often when a person hasn't had enough sleep or is trying to not fall asleep.
They're also common when in motion - such as sitting in a moving car or airplane - when your body may react to the sensation when entering the lighter stages of sleep.
Other factors that can instigate a hypnic jerk include: stress, caffeine, and alcohol.
There's also a theory that suggests hypnic jerks are the body's reaction to falling asleep much in the way that a person may twitch when dying, and the hypnic jerk is a reflex used to keep the body functioning.