Daniel Johnson:
In the Lucid Dreaming world, that's called hypnogogic imagery. It's quite common, and it is sorta a gateway to Lucid dreaming. In fact, in my own experience, the more I lucid dream, the more the differences between dream life and waking life seem to blur, and this type of imagery becomes more common in waking hours, and a sense of being awake becomes more common in dreaming hours.
I believe in the Insight world, however, it's simply another thing to notice the three characteristics of.
I agree that they are impermanent and I certainly wouldn't even know how to make them permanent if I tried.

http://en.wikipedia.org/wiki/Hypnagogia
Sights
Among the more commonly reported,[23][24] and more thoroughly researched, sensory features of hypnagogia are phosphenes which can manifest as seemingly random speckles, lines or geometrical patterns, including form constants, or as figurative (representational) images. They may be monochromatic or richly colored, still or moving, flat or three-dimensional (offering an impression of perspective). Imagery representing movement through tunnels of light is also reported. Individual images are typically fleeting and given to very rapid changes. They are said to differ from dreams proper in that hypnagogic imagery is usually static and lacking in narrative content,[16] although others understand the state rather as a gradual transition from hypnagogia to fragmentary dreams,[25] i.e. from simple "eigenlicht" to whole imagined scenes. Hypnagogia can be induced with a Dreamachine, which uses light pulsing at a frequency close to that of alpha waves to create this effect. Descriptions of exceptionally vivid and elaborate hypnagogic visuals can be found in the work of Marie-Jean-Léon, Marquis d'Hervey de Saint Denys.
It seems harmless enough. Though when you say that in lucid dreaming you feel more awake at night and less awake during the day and see hypnogogic dreams that does sound disorienting. I hope I don't have that at work.

I suppose if one were to meditate before bed these "alpha waves" are the cause.