Quote:
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. [/b]
<!--QuoteBegin-Wikipedia