The phenomenon you refer to is called "night terrors".

It has a sciencey name but I forget.
(tyrellsOwl, Fri 22 Jul 2011, 12:45, 1 reply)

It is similar to night terrors...
...but without the uncontrollable screaming. With night terrors you don't wake up until afterwards, but with this thing it wakes you up immediately.

I seem to recall it was something like "Shadow People", but a quick look on Wikipedia tells me that's not quite right either.
(anonymousreality tickled your fancy on, Fri 22 Jul 2011, 13:53, closed)

Night terrors dont always involve screaming. It is most commonly described as either
a) Seeing your room (because your eyes are still open) changing and morphing, often with demons appearing (your brain being asleep).
b) a woman/creature sitting atop you, choking you

It was this latter thing that made me think your thing might be night terrors.

In severe cases the person is almost fully awake, but unable to move, and just has to last it out.
(tyrellsOwl, Fri 22 Jul 2011, 15:26, closed)

I guess it could be then.
Fortunately it doesn't happen that often, hasn't happened for a while, which makes me think I'm due!
(anonymousreality tickled your fancy on, Fri 22 Jul 2011, 18:20, closed)