You're gently falling asleep after a long, hard day of tiring work. You're feeling very peaceful, with a big fluffy comforter and your head pressed snugly into your pillow. You can feel everything fading away whOHMYGODWHATTHEFJUCKGNINGA! Oh, DAMN! Oh, damn! Oh, man, that was scary.
You're sitting up in the pitch black, panting and covered in sweat. What the hell was that?! You hope that that will never happen again.
But it will, trust me. It will happen again. And again. And again.
A hypnic jerk is a good way to finish your day with your heart speeding at 160 bpm.