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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 paralysis happens when parts of rapid eye movement (REM) sleep occur while you're awake. REM is a stage of sleep when the brain is very active and dreams often occur. The body is unable to move, apart from the eyes and muscles used in breathing, possibly to stop you acting out your dreams and hurting yourself.