Hypnic jerk
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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. 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.
- That in the same way a body will often twitch as a person dies (as a reflex to attempt to keep the body functioning) it is thought the body might misinterpret falling asleep as a situation in which the body needs to be stimulated.
- A more radical explanation is found in "The Dragons of Eden", by Carl Sagan. This momentary conciousness and physicality are related to the time when our ancestors still slept in trees due to nocturnal ground predation. If you could be awakened enough to keep yourself from falling out of a tree, then you would presumably outlive your less evolved contemporaries, thus reinforcing this reaction over many generations of higher apes through natural selection. This also provides an explanation of the falling sensation.
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.
