When you lie down in bed, finally start going to sleep. And right then an there, right when you are LITERALLY about to fall asleep, you think you are falling off a cliff and you jolt up, wide awake, just sitting up in bed. This is actually completely normal and has to do with all of your muscles clenching together while falling asleep, then releasing all in about half a second.
I was so tired, finally falling asleep, then BAM. Just got up from a sleep start.