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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.
Sleep starts are also known as hypnic or hypnagogic jerks. They are sudden, brief and strong contractions of the body or one or more body segments. They occur as you are falling asleep. Sleep starts usually consist of one strong jerk that affects much of the body. The arms and legs are most likely to be affected. A sharp cry may also occur.