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Imagine that you're relaxed and drifting off to sleep when suddenly an arm or leg twitches and you have the sensation of falling out of bed. If this has happened to you, you've experienced what sleep specialists call a hypnic jerk or sleep start. Sleep starts happen in normal, healthy people and don't ordinarily require any medical treatment. However, certain medical conditions can make you jump in your sleep and should be evaluated by your physician.
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.