here is another article that discusses this in more depth. bottom line: it ain't no big deal.......... certainly not that sporadic fatal insomnia thing
A sleeping disorder that some people experience periodically throughout their life, but may not know much about, is something doctors call Hypnic Jerk or Hypnagogic Jerk. It's where a person drifts off to sleep and their body jerks or spasms. Sometimes these hypnic jerks cause them to bolt upright in bed. The cause of these jerks is what is referred to as a Myoclonic Twitch and it's comparable to a hiccup. Hypnic jerking is considered to be a mild sleep disorder for which there is currently no cure. Only about 10% of people 'suffer' from this sleep disorder and about another 60-70% of adults will experience at least one episode at some point in their life.
This type of sleep disorder is commonly believed to be related to sleep deprivation, stress or anxiety. Although many doctors have studied hypnic jerking, no one knows with any certainty the cause of it.
One accepted theory, however, is that it's just part of the process of falling asleep. Your body goes through many stages and cycles while falling asleep and during sleep. Brain waves change, your heartbeat slows down, your body's core temperature drops, your muscular system and nervous system go into 'sleep' mode, tissues repair themselves, and chemicals are released in your brain.
There is a sudden feeling of danger when hypnic jerking occurs though, and some theories suggest that it could be triggered by our muscles relaxing. This our subconscious interprets as a message that we're falling or drowning. The jerk is your body's attempt to 'catch' you or maybe just wake you up. Many people who suffer from hypnic jerks claim to experience a feeling of someone or something catching them right before they jerk.
These hypnic jerks can happen once or twice a night to sufferers - and they may not even realize it's happening. It's only when their partners witness it and tell them that they actually become aware of the problem. Some cases, however, have been reported where the jerks were much more severe. In these cases, the spasms come in 30 second intervals which negatively affects the sleeper and their partners life. This more serious condition is known as Periodic Limb Movement.
The is no cure for hypnic jerks. Your doctor will tell you to get more rest and exercise, and to eat healthy - the normal routine doctor advice. If you suffer from these spasms, try to avoid foods and stimulants such as caffeine or watching TV before bedtime. Of course, if your experience with hypnic jerks is serious, you should talk to your doctor.