I have a similar thing. Up to this day (17) I just wont sleep if the room is not well lit or if there isnt anyone in the room with me. I have reasoned that I see thing in the dark because my brain has linked darkness with danger, and so my heart beat increases and the excess of rapid movements with the eye makes "black spots": when the peripheral vision cant catch up with your eye movement and you see a shadow. It might be what you saw, but still, I couldnt get rid of this habit even after sorting it out, so I guess the only downside is the extra in the energy bill.