I really like this: "a generally accepted warning sign that the dawn was approaching"

This suggests some sort of intelligent community down in the underworld. "generally accepted" hints at a scientific method and scepticism, you're giving the locations of the story a nice depth, with a hint of an interesting history. Hint is the key word here. I think the use of hinting is used quite effectively in this line in particular.

"Sunrises were extremely unpleasant experiences for half-breeds"

Again, this draws in my interest. I want to know more, but dont give it to me, make me want it and read on to find out why this is.. I have to get off the computer now, ive miore to say and ill get back to you soon