A hypnic or hypnagogic jerk is the experience of a large jolt, usually felt just before falling asleep.
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 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. A series of regular jolts throughout the night (such as one every thirty seconds) may be a disorder called periodic limb movement (once known as a myoclonic jerk). The person with the disorder will usually sleep though the events, and the partner sleeping in the same bed is kept awake.
