sleep paralysis
I very recently experienced something I had only read about before, a very interesting phenomenon called sleep paralysis.
Physiologically it is related to the paralysis that occurs during normal REM sleep which prevents a person from acting out their dreams. Sleep paralysis occurs when the brain awakes from a REM state, but the body paralysis persists.
It is a very strange experience, being awake but completely unable to move, yell out, or even open your eyes. There is a very powerful sensation of being held down, accompanied by strange auditory and tactile hallucinations. I did not experience any visual hallucinations, though I have read they are often present.
I was awake, trying very hard to move or scream, but couldn't. It probably would have been terrifying if I didn't know what it is, but mostly I just thought it was interesting. It was over quickly and I didn't get a chance to focus on any single aspect of the experience. If it ever happens again, I will try to pay more attention.