Involuntary twitching or jerking when you fall asleep. Nearly 70 percent of all people have experienced a twitch or two when they fall asleep. Some are hardly noticeable, but some of the jerks can be so violent that they awaken the sleeper.
Many scientists believe that nerves misfiring as they transition between sleep and wakefulness cause the twitching. Others, however, believe that the involuntary movements are a throwback to when our primate ancestors slept in trees. If they became too relaxed during sleep, they may fall out of the tree. The twitches then evolved to protect those earlier primates from harm.