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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.
Confidence votes 118. The sensation of falling as you fall asleep is known as hypnagogic myoclonic twitch or Hypnic jerk. Most experts agree that this is a natural part of the sleeping process, much like slower breathing and a reduced heartbeat. The occurance is well known and has been well documented.