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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.
1 Stage N4: The final stage of sleep is REM sleep, or dream sleeping. 2  This occurs about 70 to 90 minutes after falling asleep. 3  You will likely experience rapid eye movement, shallow breathing, and your heart rate and blood pressure may increase. 4  During this stage, your arms and legs are also paralyzed.