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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.
And sleep apnea, where a person stops breathing intermittently throughout the night causing disrupted sleep, can also trigger you to wake up. With sleep apnea, you might not notice it unless you have a bed partner saying you're having pauses in breathing during sleep, he says.