In Shakespeare's "Macbeth," the line encapsulates the deep psychological turmoil experienced by Macbeth after committing regicide. The act of murdering King Duncan disrupts the natural order, leaving Macbeth haunted by guilt. The notion that he cannot find peace or rest after taking such a heinous action reflects the broader theme of the corrupting power of ambition and moral decay. It suggests that once someone has crossed a line, like Macbeth has, they are forever changed and can never return to their former self.

The phrase "Sleep no more" serves as a powerful mantra within the play, symbolizing not just the loss of literal sleep but also a loss of innocence and security. Macbeth's ambition has killed not only Duncan but also his own peace of mind, leading to paranoia and fear. Thus, the cry becomes a reflection of his inner chaos, highlighting the consequences of unchecked ambition and the psychological burden of guilt that can plague a person following their moral transgressions.