What are those twitches and jerks that occasionally wake us just as we are falling asleep? Dingy me wanted to know.

It has probably happened to you. You are nestled snugly under the covers, you aren’t quite sleep but you’re not quite awake. Just as your brain waves start to slow, and as you fantasize about having to see that hottie in your dream world, you are jolted awake by an unaccountable spasm, usually in a leg.

You are been a victim of what is called a “hypnic jerk”. Simply put: They occur when nerve fibers leading to the leg, in a bundle nearly as thick as a pencil, suddenly fire in unison. Each tiny nerve in the bundle produces a harsh tightening of a tiny portion of the muscle fiber that is linked to it down in the leg, and when they all fire together, the leg twitches as a whole.

Sleep experts haven’t pinned down what cases hypnic jerks or why they occur only at the onset of sleep. Although some people experience them more than others, their appearance is unpredictable unlike myoclonic jerks, spasms that occur at regular intervals during deep sleep.


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