I think he was fully conscious each and every second of each and every minute of each and every night, thinking about the events of 1795, and twisting them around until he got them into a format that he could live with: it was all Jeremiah's fault, and Angelique was just sort of by-the-way. The reason we don't hear about her until after the Vicky's trip to the past is that he just didn't want to remember that he had messed up so badly - but when Cassandra came along he had to dredge up some unwelcome memories. (I can't quite make this work, but then, I'm not a vampire.)
I see fan fiction in which he says he was in some sort of a coma or dreamlike state, and I don't buy it; it's sugar-coating the issue. He was trapped in a coffin for nearly two hundred years, and it was horrible, and that was that.
Putting it that way, it sounds positively excruciating! That's enough to turn anyone in to a raving lunatic. Can you imagine rehashing every second of your life over and over again for two centuries? Poor Barnabas!
Yes, I always wondered why before the events in 1795, Barnabas regards Jeremiah with such hatred, then in 1795 we see that Angelique manipulated both Josette and Jeremiah, and that she was solely to blame for everything that happened, including the disintegration of Barnabas's and Jeremiah's close relationship.
but it makes sense, that if after a while of being imprisoned in the coffin, Barnabas would start to mull things over and turn reality in to a fiction he could cope with. And after nearly two hundred years of this, it's no wonder he's a bit unbalanced as evidenced in all the strange things he did upon his arrival at Collinwood.
I'll say it again, poor Barnabas!