A hypnic or hypnogogic jerk is an involuntary muscle twitch (more generally known as myoclonus or a myclonic twitch) which often occurs during the transition from wakefulness to sleep (see hypnogogia).
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.
The exact cause of the sudden jerks is not clear, but there are two main hypotheses to explain them. One is that it is a naturally occurring part of the sleep process, as are slower breathing, and reduced body temperature. Another hypothesis is 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.
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 much 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, and the partner sleeping in the same bed is kept awake.