The Dracula of Stoker's novel is quite a different figure from what he has morphed into, most especially with the Balderston play from which the original Lugosi/Universal film was adapted. In some ways the controversial Coppola version is much more accurate, but oddly contains an introduced subplot that made Mina into a rather Josette type figure. While many people pointed to the Dark Shadows influence as having been filtered down through Dan Curtis's Dracula adaptation, I think if you're going to add this element, Mina is a better Josette surrogate than Lucy as in the Curtis version. Honestly, the root of this subplot is really found in the original Karloff film, "The Mummy."
I prefer the concept that vampires in the day are in a "natural" state of death. I also tend to agree that at night, if trapped in the coffin, they would have some sense of the passage of time, but I wish I could remember the line Frid did so eloquently about 'winds of time sweeping past'. I would think this would be further altered by the placing of the cross in the coffin's upper lid. I'm sure someone here also remember how Anne Rice's novels refer to this, so I will beg their remembrance, as at this point its better than mine. I think it would be in a more altered state, a supernatural come so to speak.
I always note the contradiction of Barnabas moaning about spending "years in this state" when in actuality, he was really only active as an undead for maybe a few months before Joshua chained him in. This is very unlike Count Dracula who does appear to have been an active vampyre for centuries when the novel begins.
Michael