Hypnic jerks

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.

Cause
The exact cause of the sudden jerks is not clear, but there are hypotheses:
 * That it is a naturally occurring part of the sleep process, as is slower breathing, and reduced body temperature.
 * That the feelings associated with relaxation before sleep are misinterpreted by the brain as falling or toppling over, and by reflex the body jerks out the limbs to stay upright.

Occurrence
Hypnic jerks are usually felt just once or twice per night. More regular (and usually less dramatic) muscle twitches often occur during normal sleep (perhaps as often as one every thirty seconds). 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.

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 successfully deprived themselves of sleep for longer than 24 hours.