Have you ever been startled out of your sleep by an involuntary jerk of your arms or legs? According to researchers, this phenomenon is known as the "hypnic jerk." It happens as you are drifting off to sleep, and your muscles are deeply relaxing. There's no widely accepted explanation for the jerk, but it's likely that it's caused by a misfired message from the brain that stimulates, rather than relaxes, a given group of muscles.