A hypnic or hypnagogic jerk is an involuntary muscle twitch (commonly known as a myoclonic twitch) which occurs during the transition into hypnagogia.
It is often described as an electric shock or falling sensation, and can cause movement of the body in bed. Hypnic jerks are experienced by most people, especially when exhausted or sleeping uncomfortably.
Hypnic jerks are usually felt once or twice per night. More regular, and usually less intense, hypnic jerks often occur during normal sleep. In extreme cases, this may be classified as a disorder called periodic limb movement. The person with the disorder will usually sleep through the events.
Although the ultimate cause of the hypnic jerk is unknown, a common hypothesis is that the brain misinterprets relaxation as the sleeping primate falling out of a tree.
When a subject is deprived of sleep and is trying to fight sleep, hypnic jerks can occur more often. This normally happens to subjects who have deprived themselves of sleep for longer than 24 hours, or to those who have recently woken up from insufficient amounts of sleep.