I get these more than ever…
A hypnic or hypnagogic jerk is an involuntary myoclonic twitch which occurs during hypnagogia, just as the subject is beginning to fall asleep. Physically, they resemble the “jump” made when a person is startled.
Often accompanied by a falling sensation, it is commonly caused by irregular sleep schedules.
Origins
The neurological reason that hypnic jerks occur is not fully understood, although there are two predominant theories.
Some researchers suggest that as a subject’s heartbeat and breathing slow down, hypnic jerks occur as a natural part of muscular transition.
Another theory states 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 – and triggering a response to thrash the limbs in an attempt to catch something or turn oneself upright.
Occurrence
Hypnic jerks are usually felt once or twice per night. More regular, and usually less intense, hypnic jerks often occur during normal sleep. 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 deprived themselves of sleep for longer than 24 hours, or to those who have recently awakened after insufficient sleep.
I learned in a psych class in college that it was caused by some chemical in your nerve receptors. I can’t remember the chemical, but I think it was a result of all of it leaving at once, causing the nerves to make your muscles jerk – almost like a shock.