edit on 24/3/2011 by Griffo because: (no reason given)
A special form of pre-sleep imagery, called HYPNAGOGIC IMAGERY, is experienced by about 33% of people. To see if you can experience those, simply try to be aware of what you see as you fall to sleep.
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Hypnagogic images, according to some, are the result of the visual system "attempting to make sense" of the visual input still available to the eyes when the mind is drifting toward sleep. Even in very low ambient light levels, light penetrates the eyelids, making the patterns of blood flow through them visible. This is the basis for an "interpretation" by the visual system. The visual system may attempt to "force" the input to correspond to, say, a geometrical pattern, or the semblance of some object. Dream images (another type of hallucinogenic image) or spontaneous memory images (another type of mental image) may intrude into such images. The stages, or types of imagery, associated with sleep and dreaming share an important common characteristic that figures in their definition: they are autonomous.