What are those twitches and jerks that wake us just as we are falling asleep?
It probably has happened to you. You are nestled snugly under your blankets. Just as your brain waves start to slow, and as you fantasise about striking the TOTO jackpot, you are jolted awake by a spasm, usually in the leg.
You have beena victim of what is called a 'hypnic jerk', a phenomenom explained in David Bodanis's The Body Book:
"They occur when nerve fibres 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 muscle fibre that is linked to it down in the leg, and when they all fire together the leg twitches as a whole".
Sleep specialists haven't pinned down what causes hypnic jerks or why they occur only at the onset of sleep. Although some people experience them more often that others, their appearance is unpredictable, unlike myclonic jerks - spasms that occur at regular intervals during deep sleep.
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