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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. Sleep paralysis is a phenomenon in which a person either during falling asleep (Hypnagogia) or awakening (Hypnopompia), temporarily experiences an inability to move, speak, or react.It is a transitional state between wakefulness and sleep, characterized by muscle atonia (muscle weakness).hysiologically, sleep paralysis is closely related to REM atonia, the paralysis that occurs as a natural part of REM (rapid eye movement) sleep. Sleep paralysis occurs either when falling asleep, or when awakening from a session.