Things start to get hairy for the narrator when the second cat comes along. The cat won't leave him alone, day or night. If the man falls asleep, he has bad dreams, and always wakes up with the cat sitting on his chest, breathing on his face. So the narrator eventually stops sleeping.
The narrator describes the cat as a "Night Mare," though some texts, like the University of Virginia e-text used here, run the two words together to form nightmare, which is the usual contemporary spelling. According to a footnote in The Selected Writings of Edgar Allan Poe, "The Night Mare myth was a dream horse ["mare" is another word for "horse"] that trampled people in their sleep, it's great weight causing a sense of suffocation" (353, Source ).
In many Poe stories, we aren't completely sure whether the narrator is asleep, awake, or somewhere in between. "The Black Cat" is one of those stories. The narrator admits to nodding off frequently, and to sleep deprivation. His dream life and his waking life combine to form an almost seamless nightmare-scape.
As with all his other problems, the narrator blames this situation on the cat. In his old cat-lover days, he might have considered the cat's snuggling a sign of affection, but the cat has become an easy victim for his rage. He sees it as a sign of menace, and of his guilt. It is only once the cat (and the wife) are out of the way, the man sleeps easy.