Myoclonus

These are the involuntary twitches produced while sleeping. Check the following out for more information

The straight dope page

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Skinny

The upshot is that no one knows why they (the more common hypnic jerks) happen. A simple minded explanation is that as one sinks into sleep the muscles begin to relax and lose "tone". The brain interprets this as "falling" or "dying" and attempts to "revive" them by sending bursts of impulses. But, i guess this must be simplistic because the whole sleep phenomenon must also be seen by the brain as "dying" - then how come it "allows" to sleep at all...

The more serious ones (hypnagogic jerks) are seemingly due to brain problems.