Quote:
A hypnic jerk, hypnagogic jerk, sleep start, or night start, is an involuntary myoclonic twitch which occurs during hypnagogia, just as a person is beginning to fall asleep, often causing him or her to awaken suddenly. Physically, hypnic jerks resemble the "jump" experienced by a person when startled,[1] often accompanied by a falling sensation.[2] A higher occurrence in people with irregular sleep schedules is reported.
There are countless threads regarding this already on DV, so...I'm just gonna close this since it doesn't really add any new information or bring up any issues that need to be discussed, and the question was answered already. :P