Is consciousness present during deep sleep? I would have to say no to that question, but tentatively.
I think that ultimately we can't know with complete certainty, as it is possible that our memories are erased once we wake up and we really do just stare into the void for an average of 8 hours, for me 3.
But certainty is not necessary for knowledge.
If we go with it from a purely a priori standpoint, it's probably more likely than not that we are not conscious during that time. Just from phenomenological conservatism alone.
If we want to use Ockham's Razor, then we would have to argue whether or not it is more simple that our consciousness ceases or not. If I were to just examine the two claims point-blank, then I would probably say that both are equally complex, so our justification for both would be equal.
Now, I think it's best to appeal to some empirical data here, not that I'm an empiricist, necessarily. And see what the whitecoats have to say about this matter.