After killing Duncan, Macbeth becomes haunted by his actions. He imagines hearing a voice that says he has "murdered sleep," meaning he has destroyed his own peace and innocence. This symbolizes how the murder has robbed Macbeth of his ability to rest and find peace, foreshadowing his increasing paranoia and insomnia throughout the play. Sleep becomes a metaphor for innocence and peace of mind, both of which Macbeth has now lost.