Disturbing nightmares

You know those things that don't scare you, yet at the unright time they come, and its, disturbing even through you know they aren't real, you can't control your own mind in those fear circumstances such as nightmares, they are annoying and don't let you concentrate.
They are allied with your imagination, the greater it is, they stronger they are, yet if you put it into a good use, they can be easly defeated.

This disturbing times mostly come from the dark, yet there can be another cases, because you can't see that well, and associate what you see with something, bad, as your brain is a bit frightened, why its frightened? well it can be for many reasons, but mostly simply because you can't recognize something you can't see.

Yet there is nothing worse than to think that you are dreaming then awaking up and realizing that you just wake up to a even worse nightmare.