Have you ever had a dream where all of a sudden you feel like your falling, and almost immediately wake up because of it, feeling like you’ve an intense muscle spasm with your heart pounding? This phenomenon is called hypnic jerk, and if you’ve felt this before you’re not alone, more than 70% of the population experiences once in their life. A hypnic jerk is an involuntary twitching of a muscle, or multiple muscles, and they usually occur just after you fall asleep. The reason you feel a sensation of falling or jolting, is because your muscles involuntarily twitch as you pass through different phases of sleep.

http://www.nosleeplessnights.com/the-hypnic-jerk-jolted-awake-when-falling-asleep/