The jump you sometimes get while falling asleep has a name!
Falling asleep after a long day of work or school? One of the most awkward sensations you can ever feel while falling asleep is feeling that you are falling down.
A hypnic jerk is an involuntary twitch which occurs during hypnagogia, just as a person is beginning to fall asleep, often causing him or her to awaken suddenly for a moment.
Physically, hypnic jerks resemble the "jump" experienced by a person when startled, often accompanied by a falling sensation. A higher occurrence is reported in people with irregular sleep schedules.
According to the American Academy of Sleep Medicine there are a wide range of potential causes, including anxiety, caffeine, stress and strenuous activities in the evening.