Why do we twitch while falling asleep?

Dear Cecil: Occasionally, just prior to falling asleep, I have experienced a jerk or twitch, as if my body is trying to reverse its inevitable slide into unconscious slumber. How do you account for this peculiar behavior? It’s also been known to happen to me when drowsiness overcomes me while listening to a particularly dry lecture. H.G., ChicagoDear Cecil:Why do we itch? I don’t mean the specific itches of a mosquito bite, a healing scab, etc.; I mean why do an itch suddenly arise on a finger or your back or someplace for no particular reason? It may go away with a light scratch; it may persist for minutes in spite of scratching. Is there a simple physiological explanation for this? Dawn O., Evanston, Illinois

Cecil replies:

Nothing is simple by the time I get done with it, Dawn. Unfortunate as it may seem, medical science does not fully understand any of the phenomena described above. However, it has composed impressive polysyllabic names for them, which is surely the next best thing.

Twitches while falling asleep are called hypnagogic myoclonus, myoclonus being any sort of involuntary muscle spasm and hypnagogic referring to sleep. The twitches occur during very light sleep as the conscious brain gradually relinquishes control of the motor functions. Often they’re accompanied by a sense of falling, or the feeling that something is flowing through the body, and sometimes people will experience vivid dreams or hallucinations.

It’s not known exactly what causes the twitches, but they appear to be associated (although by no means invariably) with (a) anxiety and (b) some faint stimulus, such as a noise. The twitches have been induced in test subjects who were instructed to push a button whenever they heard a low tone. When, as usually happened, the subjects nodded off after a while — you know how exciting psychology experiments are — the tone would often cause a subject to twitch after a lag of a few seconds.

It’s conjectured that the subjects consciously knew they were supposed to stay awake, that they fell asleep anyway, and that the tone jarred the semiconscious brain into trying to scramble itself into action again. That would explain why you experience the twitches during boring lectures. It’s thought that at times the stimulus can be purely involuntary, such as a dream.

The general medical term for itching is pruritus (proor EYE tuss). There are all kinds of pruritus; the kind we’re talking about here is called punctate pruritus, spot itching not triggered by any obvious skin disease or other cause.

The operation of the nerve endings in the skin is not clearly understood, but itching appears to be associated with the sense of pain, since persons who can no longer feel pain, for whatever reason, usually don’t itch anymore either. In this respect itching is analogous to tickling, which is thought to be related to the sense of pressure.

There are numerous “itch points” scattered about the surface of the body where it’s possible to induce itching simply by touching with a fine metal wire. Other areas on the skin usually are relatively insensitive. Itch points appear to be associated with concentrations of fine free nerve endings.

It’s known that in some cases the nervous system has different mechanisms for conducting sensory impressions of varying intensity, such as light and firm pressure, and there is speculation that itching may be a watered-down sense of pain, designed to detect extremely faint stimuli. Thus if you have dry skin or a stretched hair follicle or some minor localized chemical imbalance, the nerves may multiply it into the sharp irritation that you instinctively scratch. Then again, who knows?

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.