In Act 3, Scene 4. when Macbeth sees, or thinks he sees, Banquo sitting in his place at the banquet table he displays acute paranoia. He asks the other guests, "Which of you have done this?" He is obviously mentally unbalanced. His behavior at the banquet seems to foreshadow how he will behave for the rest of the play. He is frightened, but he cannot permit himself to acknowledge his fear. His behavior betrays his guilt to the entire assemblage, and will cause him to become abandoned and isolated. Nothing his wife can do or say can persuade him to stop interacting with a ghost whom only he and the audience can see.
After murdering Duncan, Macbeth tells his wife:
Methought I heard a voice cry "Sleep no more!"
Macbeth does murder sleep"
And a few lines later:
"Glamis hath murdered sleep, and therefore Cawdor
Shall sleep no more. Macbeth shall sleep no more."
It might be assumed that Macbeth's steady mental deterioration is due to the fact that he literally is unable to sleep from that fateful night until the end of the play--a horrible but richly appropriate punishment for his crime. It seems significant that Lady Macbeth seems to be suffering from the same insomnia, judging from her haggard appearance and mad soliloquy in Act 5, Scene 1. There is a nightmarish quality about the entire play of Macbeth.