Yael: Last night as I was drifting off to sleep, my leg suddenly twitched so violently that I      nearly kicked my husband out of bed!

Don: Sounds like a hypnic jerk.

Y: Who, my husband?

D: Not your husband, Yael, your leg twitch! The twitches or jerks that sometimes      happen when you're falling asleep are called hypnagogic myoclonus, or hypnic jerks.      They are normally caused by sudden muscle contractions; but they also can result      from brief lapses of contraction in a normally active muscle.

Y: Are hypnic jerks normal? Why did I do it? Does it mean I secretly don't like my      husband?

D: Well, I don't know about your feelings for your husband. But typically, hypnic jerks      are a normal part of a body's transition from alert and awake, into sleep.

     Scientists don't know for sure why they happen, but there are at least two      hypotheses.

     The first is that the jerking motion is simply part of the normal physical changes that      happen when you fall asleep -- your breathing slows, your temperature drops, and      your muscles relax. The relaxation of one muscle can often cause a sudden jerk in the      opposite direction.

     A second hypothesis is that your brain interprets muscle relaxation as a signal that      you're falling, and reacts by sending nerve impulses to your limb muscles to try to      stay upright. This hypothesis makes sense to many people since hypnic jerks are      often accompanied by sensations or visions of falling as one drifts into sleep.

     So normal hypnic jerks are nothing to be concerned about, they are just annoying,      like most jerks.  

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