Re: Why do you begin to twitch just as you fall asleep (neurologically)?

Hi, A.B.!

That's a very interesting question! I think what you're asking about are referred to as hypnagogic massive jerks, also known as hypnic jerks, predomital myoclonus, or (more commonly) sleep starts. First described over 100 years ago (Mitchell, 1890), sleep starts are generally sudden, brief contractions of the muscles in the leg, and sometimes the arms or head at the onset of sleep (see also Oswald, 1959). Although mostly reported by the partners of the "sleep starters", about 10% of the population report experiencing them regularly, 80% report occasional sleep starts, and the remaining 10% report rare sleep starts (Oswald, 1962). Unfortunately, relatively little research is directed at this phenomenon, so not much is known as to it's cause, correlates, or consequences (although there are some results available from a rather nice little correlational study here).

There are some physiological correlations, however, that may be made to a certain extent via electroencephalogram (EEG) readings during sleep onset. EEG recordings measure the net average of all neuron potentials at a given time. This recording will only register (by a rise or fall) when a number of neurons fire simultaneously. It helps to compare it to a noisy crowd at a sporting event…you only hear a change in the level of noise when a whole bunch of people shout at the same time (like after a goal or a bad call). When someone is falling asleep, the EEG tends to read steady "waves" of activity (about 10 per second, called alpha waves), which then decreases and slips into small, irregular, jagged theta waves as stage 1 sleep is entered.

During a sleep start, however, the EEG picks up a very sharp "peak" of activity. Afterward, sometimes, the EEG records an increase in alpha wave activity (Oswald, 1959). This outcome isn't really surprising, but it does demonstrate a surge of action potentials at the time of the sleep starts. Exactly which neurons are doing all of this firing, however, is pretty much unknown although there is some data related to a similar progressive movement disorder (myoclonus) implicating neurons associated with the neurotransmitters serotonin or GABA.

I hope this shed some light on a rather under-explored subject. Though no changes have been unequivocally established to cause this surge in action potentials, it has been established that sleep starts can be provoked by external stimuli (Oswald, 1962). Also, the site that I have linked above records some rather interesting correlational data from self-reports, indicating some possible antecedents (e.g., fatigue or stress).

Feel free to return if you have any more questions!
Dr. Tom Virden.

References

1. Mitchell, SW (1890). Some disorders of sleep. Am. J. Med. Sci. 100:109-127.
2. Oswald, I. (1959). Sudden bodily jerks on falling asleep. Brain. 82:92-103.
3. Oswald, I. (1962). Sleeping and waking: Physiology and psychology. Amsterdam: Eselvier.
4. Snodgrass, SR. (1990). Myoclonus: analysis of monoamine, GABA, and other systems. FASEB J. 4:2775-2788