Startled Awake in Sleep

Sleep start. Ever try falling asleep before realizing that you were actually falling? So that when you realize you're falling you try to move your legs and arms-- but then realize you can't? And moments after you whole body starts twitching? I have. And apparently this is called sleep start. Except for me what happens would be I'd have a nice night of sleeping then I'd wake up in the morning completely aware of my dog curled up into a ball on the pillow next to me and the time of the alarm clock-- without even opening my eyes. It's somewhat like just sensing. Then in the midst of it all I feel like I'm falling or dying and I can't help but try to scream-- only to not be able to move my mouth. All of a sudden it's like I'm being constricted and I keep twitching, unable to move or scream or do anything. Seconds later, it passes and I lay there, my heart pounding in my chest while uneven breaths escape me. The whole ordeal has lasted only about ten seconds, it felt like hours to me.