Originally Posted by
Robot_Butler
There is one additional explanation for why people often have the common experience of seeing a presence in their room when they are on the verge of sleep. Theoretically, if you are dreaming, you could hallucinate anything. Why would there be a commonly experienced hallucination?
The reason people see Old Hag is because humans have a blind spot in the center of their vision. During normal, every day life, your brain fills in this blank area before your mind interprets the image. You never see the blind spot. When you are just waking from or slipping into a dream (especially in a dark room) your mind will place erroneous images into this blind spot. You do not have enough visual information to fill in the blank spot, so you fall back on images from the dream.
This is why many people report seeing ghost images when it is dark, or when they have just woken up. Even if you are fully awake, the hallucinations can still appear. They often seem to be indistinct, gray or shadowy floating masses. They commonly seem out of scale with their surroundings, or incongruous with your depth perception. As you move your eyes around the room, the image will stay rooted in the center of your vision, so will appear to float. If you've seen it before, you know what I mean.
If you want to experiment with this, just sit in a very dark room and stare at a certain point in space for a few minutes. It works especially well when you sit in front of a mirror and stare at a barely visible image of yourself. You starve yourself of visual input, so your brain starts going haywire trying to dump something into the blind spot.
We try to make this image make sense, which can lead to all sorts of strange hallucinations. We change what we see to try and fit this anomaly. The hallucinations can get quite involved, with the false images interacting with our surroundings or even our bodies. If you are interested in this, read about Charles Bonnet syndrome. Just google it and go to town.
Here is a quote that explains it well in relation to sleeping in a dark room: