Hypnic jerk
A hypnic jerk, hypnagogic jerk, sleep start, or night start is an involuntary myoclonic twitch which occurs during hypnagogia, just as the subject is beginning to fall asleep. Physically, hypnic jerks resemble the "jump" made when a person is startled,[1] often accompanied by a falling sensation.[2] It is commonly caused by irregular sleep schedules.[3]
[edit] Origins
The neurological reason for hypnic jerks is not fully understood, although the two dominant theories suggest that as a subject's heartbeat and breathing slow down, hypnic jerks occur as a natural part of muscular transition;[4] or that as a subject falls asleep, their muscles begin to relax and cease working, causing the brain to believe that the body must be falling through air. It is thought that this causes people to thrash their limbs in an attempt to catch something or turn oneself upright.[4]
[edit] Occurrence
Hypnic jerks are usually felt once or twice per night, up to a few times a month. More regular, and usually less intense, hypnic jerks often occur during normal sleep. In extreme cases, however, this is classified as a disorder called periodic limb movement. People with the disorder usually sleep through these events.
When a subject is deprived of sleep and is trying to fight sleep, hypnic jerks can sometimes occur more frequently. This normally happens to subjects who have deprived themselves of sleep for longer than 24 hours, or to those who have recently awakened after insufficient sleep.
