It has probably happened to you. You are nestled snugly under the covers. You aren't quite asleep but you're not quite awake. Just as your brain waves start to slow and as you fantasise about owning that Mercedes Benz convertible, you are jolted awake by an unaccountable spasm, usually in a leg.
You have been a victim of what is called a 'hypnic jerk', a phenomenon explained in David Bodanis's marvellous 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 have;t pinned down what causes hypnic jerks or why they occur only at the onset of sleep. Although some people experience them more often than others, their appearance is unpredictable, unlike myoclonic jerks, spasms that occur at regular intervals during deep sleep.
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