The body jerks because it feels like you're falling, and your brain sends out a reflex twitch signal to try to pull you back form the brink. The technical name for this is hypnic jerk, or hypnagogic myoclonic twitch.
You feel the falling sensation because your brain literally disengages from the body while you're asleep. This prevents you from moving around too much in your sleep, especially if you have very vivid dreams. Of course we can still move in our sleep, but it takes a lot of stimulus (and usually coming close to being awake again) to overcome the barriers to most muscle movement put in the way by the brain.
Some people don't get a "falling" sensation. It's more like floating for them. In which case, their brains sometimes act to pull them back down to earth. Either way, the lack of contact with the body and where the body is in space is very disorienting for the brain.
Hypnic jerks aren't harmful, although they may wake you up again.
This I have done research on, and is quite normal, but can be the scariest thing in your life if you don't know what's happening.
What is happening is when you start to feel a falling sensation, you have passed into a zone of subconsciousness. In this stage your brain thinks you're asleep but you are really awake. You therefore will hear and feel things that aren't real, but they are real to you as the dinner you just ate. It is a dream in real life so to speak. Most parts of the brain have shut down, but not all like normal sleep. The part that makes conscious decisions, such as to think of the word pie, is still working, and usually so are the eyes. However, if you try to scream, you will find yourself only to be able to let out a soft moan, and that's with all of your might. Most people wake up before they reach this stage of hallucinations, which is good. However, this started for me when I was about 10, and I had three terrifying hallucinations (in three different cases) before I was able to wake up. I had to train myself.
The three hallucinations I had were quite like dreams, but they were real. The only thing is they were only real to me, because my brain was pretending by sending out false impulses to my sense receptors, so it feels and is real life, but only to you. It is like a dream - you can program it to your wish, you just can't get out of bed.
My first one was this - I was laying in bed with my dad, when all of a sudden I felt a falling sensation. At first I woke myself up. But then I went back to sleep and felt it again. This time I found myself curious, so I followed it. The next is hard to explain, you must have been there to understand it, but I felt tingly and heard music I liked for a few seconds, so I went further into it ... 'down the tunnel' if you will. Then the good ended, and a flash of light brighter then any I had ever seen before happened and what sounded like a million guns going off, all right next to my ear, all simultaneously. I thought aliens, and I sat still, thinking they had killed my dad and would get me next. But you have to understand, I didn't think it, I KNEW it. I sat still as a rod hoping they wouldn't get me next. And, then in a creaky voice that was probably male, it said something along the lines of "There's no more." The next thing was unclear. I felt like they went under the bed. I stood still more then three hours in that bed, even afraid to breath too loud. You might think of it as an overreaction, but I can still not impress on you enough this is not like a dream's atmosphere. You feel awake and well. Of course, I told my living dad the whole story when I got up.
The second time this occurred is the most vivid of them all. I was at my mom's house trying to go to sleep when the sensation of falling happened. But it was not like the first time - there was no choice of waking up, no music, nothing. Just felt like falling off of the top of the Empire State Building. Then it stopped. Let's call this the 'stopping point.' Then it felt like something was holding my to legs (I was on my side) and spreading them apart, trying to rip them out of their sockets. I could control my vision, but no muscles or even my head. As hard as I tried, all I could do was open and close my eyelids. I woke up.
The third time was the shortest. After the 'stopping point' I felt like my bed was jiggling, like a vibrating pen, and I was too. I tried to scream, but that only let me roll out a soft bass moan. Then I woke up.
Since the third time of hallucinations, I have had many times of the sensation of falling. All I do is wake myself up and try again. Usually it will bring you back one more time before you can finally rest in peace. If you ever get scared, just remember that this is completely natural and your brain is just confused. It's a dream in a realer atmosphere.
ALL OF ABOVE IS BY: Ezra Lesser aka Biblicalreader
