When we sleep (particularly during REM sleep), our brains temporarily paralyze us by releasing acetylcholine. This is referred to as “atony”. The purpose of this is to keep us from acting out our dreams.
Sometimes, a person can become conscious while in this state and he will wake up feeling paralyzed. This is referred to as Awareness during Sleep Paralysis (ASP) or simply sleep paralysis. Because it can be accompanied by feelings of suffocation or being strangled and in the past was thought to be brought about by an old hag (or witch, ghost, demon, etc.), it became known as the Old Hag Syndrome or Old Hag Phenomenon. When a person is experiencing this, he will essentially be dreaming while he is conscious with open eyes.
If you go here, you can read about different experiences people have with sleep paralysis under “hallucinations”. Some of the symptoms experiencers may have include hearing strange sounds, laugher, voices, or footsteps. Sometimes they will feel the presence of something malevolent standing over them (the figure is sometimes described as looking, watching, or even staring), or they will actually see a shadowy figure standing over them. On rare occasions, a person will feel as if a family member or friend is in the room with them. People also experience feelings of levitation, or falling, or even sinking into their pillows or mattresses. Some believe this explains modern day accounts of alien abduction.
The reason I’m writing about this is because this is something I’ve experienced regularly since I was in the fourth grade, and it’s very interesting to me. My experiences are usually not as extreme as many accounts I’ve read or heard, but it’s annoying nonetheless. Usually I wake up paralyzed with what sounds like white noise buzzing in my ears, or I can tell I’m slipping into sleep paralysis when I’m trying to go to sleep because I feel my head sinking into my pillow or I hear white noise. There have been rare occasions where I’ve felt (or even seen) someone or something watching me, but as I’ve learned more about this, my experiences have become more of an annoyance than something to be feared.
*The image is a painting called The Nightmare (1781) by Henri Fuseli. The history of the word “nightmare” is interesting as well. The word has it’s roots in the phenomenon of sleep paralysis.
Andrew Thomasson 
I have had experiences like these, about four or five times throughout the life I have lived thus far. It can be quite a frightening experience. I cannot imagine what it might be like to be a regular. Thanks for sharing.
Note: ACh is a neurotransmitter common at neuromuscular junctions. The inhibition of ACh or the enzyme acetylcholine esterase may more likely result in paralysis rather than the release of more ACh itself by the brain.
Thanks for the comment, and thanks for the info on ACh!
I was a psych major, and I did well in all my classes except for Psychobiology. My teacher dressed, spoke and acted like someone who smoked pot all day long. She didn’t make much sense. And the subject was a bit boring to me.
omg!! my head sinking into my pillow is the scariest part of my sleep paralysis! i too get the ringing in my ears and I cannot breathe. it feels as tho if I take a deep breath, i will die. i feel like someone or something is in the room with me, mostly under my covers with me and I can feel it moving around.
what a crazy crazy phenomenon to have to live and relive several times a month. i absolutely hate it.
http://www.panicmonster.com