I've long been interested in dreaming and there's a peculiar phenomenon of the very start of the dreaming process. Different people experience it differently, but even for the same person there's anecdotal evidence of different experiences during dreaming onset.
One of the most dramatic examples is the "wake induced lucid dreaming" transition, where the subject experiences intense hallucinations. These could be sight, sound, feeling of presence, falling, buzzing, etc. As these hallucinations subside, they are replaced by dream imagery.
There are other kinds of transitions as well.
What interests me is if these phenomena have been studied using modern neuroimaging techniques. Does science know what happens in the brain when the persons perception shifts from the real world to dreaming mentations?