I knew that he had been lying awake ever since the first slight noise, when he had turned in the bed. His fears had been ever since growing upon him. He had been trying to fancy them causeless, but could not. He had been saying to himself—“It is nothing but the wind in the chimney—it is only a mouse crossing the floor,” or “It is merely a cricket which has made a single chirp.” 

Houses can make strange sounds at night. They are almost like living creatures. They settle just a fraction of an inch, and the timbers make a creaking noise. The wood tends to expand or contract with the temperature and the weather. It can pull on the iron nails just a tiny bit and make eerie sounds, sometimes like the hinges of an opening door. We lie there wondering what that was and whether we will hear it again. We would like to go to sleep, but we are afraid to lose consciousness. Going to sleep means trusting the whole world to leave us alone. It is easy to imagine that a squeaking timber is a footstep on the stairs. And then it is easy to go further and imagine an intruder creeping up on us with evil intentions. Samuel Taylor captures this feeling perfectly in one of the stanzas of "The Ancient Mariner":

Like one that on a lonesome road
Doth walk in fear and dread,
And having once turned round walks on,
And turns no more his head;
Because he knows, a frightful fiend
Doth close behind him tread.

In the old man's case, the frightful fiend is all too real. The madman who is telling this story corresponds to our own worst fears. He does just the sorts of things we can imagine when we are lying awake in the dark listening. If there is no sound we imagine the maniac is watching and waiting. He wants us to be satisfied that there is no danger. He is waiting for us to turn over in bed and close our eyes. Then, like the narrator of "The Tell-Tale Heart" he will spring.

The old man's hour had come! With a loud yell, I threw open the lantern and leaped into the room. He shrieked once—once only. 

Even while lying awake in terror we often finally fall asleep and wake up the next morning with the sun shining and realize that it was nothing but our imagination. 

For some unknown reason, the narrator is bothered by the old man’s cloudy, pale blue eye, which has incited madness in the narrator. Whenever the old man looks at him, his blood turns cold, and he becomes determined to kill him to get rid of this curse. The narrator continually argues he is not mad, yet for a whole week, he sneaks into the old man’s room every night. The problem is the victim is always sound asleep with his eyes closed, and the narrator cannot bring himself to kill the man without seeing his ‘‘Evil Eye.’’ Finally, on the eighth night, the man springs up and cries ‘‘Who’s there?’’ In the dark room, the narrator waits silently for an hour. The man does not go back to sleep; instead, he gives out a slight groan, realizing that ‘‘Death’’ is approaching. Eventually, the narrator shines his lamp on the old man’s eye. The narrator immediately becomes furious at the ‘‘damned spot,’’ but he soon hears the beating of a heart so loud that he fears the neighbors will hear it. With a yell, he leaps into the room and kills the old man. Despite the murder, he continues to hear the man’s relentless heartbeat, which continues to drive him further into the depths of insanity.