Hypnagogic imagery occurs during the nodding
off stage, and can vary considerably, from
startling warnings and vivid precognition, to
static scenes. Sometimes voices are heard, even
entire conversations. In sleep research
generally, the hypnagogic state is perhaps
underestimated.
Researchers tend to accept that it represents
the first stage of sleep, but are more
interested in other sleep phenomena. This
underpins the point that, as far as knowledge
about sleep and dreams is concerned, we are
merely scratching the surface. Within the realm
of hypnagogic imagery lies a mysterious world
that, if better understood, can offer
considerable rewards.
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 or memory images may
intrude into such images.
Precognitive visions seen during hypnagogic
imagery are said to be particularly reliable
and rarely need interpreting because, usually,
they closely resemble future events fairly
accurately.