Dexter's Darkness

Here is an exerpt from the first book in the Dexter series by Jeff Lindsay (yes, this is the same character who inspired the Dexter from the Showtime series, although the novels came first and are quite different in various ways). This is a scene from the first book in the series (ironically?) called Darkly Dreaming Dexter, in which Dexter catches himself having dreamed:

Hello, dear boy, so good to have you back. But where on earth have you been?

That, of course, was the question. I have spent most of my life untroubled by dreams, and for that matter, hallucinations. No visions of the Apocalypse for me; no troubling Jungian icons burbling up from my subconscious, no mysterious recurring images drifting through the history of my unconsciousness. Nothing ever goes bump in Dexter's night. When I go to sleep, all of me sleeps.

So what had just happened? Why were these pictures appearing to me?