What is ironic about Lady Macbeth's state of mind in Macbeth, Act 5, scene 1?
Irony generally means the opposite of what is expected. The irony, in this instance, lies in Lady Macbeth's actions. She sleepwalks and has to constantly have a light with her. Firstly, it is her guilt which keeps her awake and secondly, she seems to be afraid of the dark, since she fears the malice it might hold. Earlier in the play, though, she had assumed an aggressive and determined stance.
Lady Macbeth had, in Act l, already made up her mind to commit the direst evil and had asked the dark powers, in Act l, scene V, to remove all her feminine qualities and imbue her with the remorseless will to commit the foulest of deeds. She called on darkness so that her evil would not be seen, but here, in Act V, scene l, she is overwhelmed by remorse and unable to sleep. Her conscience keeps her awake and she is fearful of the dark, as her gentlewoman observes and reports to the doctor:
Lo you, here she comes! This is her very guise;
and, upon my life, fast asleep. Observe her; stand close.
Why, it stood by her: she has light by her
continually; 'tis her command.
Further irony lies in the fact that she was highly critical of her husband when he told her that they 'will proceed no further in this business,' a reference to the plot to kill Duncan. She is insistent and very persuasive, so much so that her husband relents and decides to continue. When he later asks what should happen if they fail, she responds by telling him that he should gather his courage, for that is all that they need to ensure success.
In Act ll, scene ll, It is also ironic that she finds Macbeth's concern about not being able to say 'Amen' after Duncan's execution as a trivial matter that he should dismiss since, if they give it too much thought, 'it will make us mad.' She now, in Act V, though, is the one overwhelmed by guilt and is obsessed by thoughts of the murders of Duncan and Banquo. She is so overcome that she loses her mind. Her desperation drives her to suicide whilst Macbeth has retained his sanity and has not entirely lost his composure.
Furthermore, she also finds Macbeth's references to visions, such as Banquo's ghost, laughable and just a symbol of his cowardice, but now she herself imagines seeing bloodstains on her hands which she believes cannot be removed. She states, for example, that:
all the perfumes of Arabia will not sweeten this little hand.
However, soon after they had murdered Duncan, she told her husband that he should wash the blood off his hands, for 'a little water clears us of this deed.'
In the end, Lady Macbeth's torment becomes too unbearable and she takes her own life while her husband—who, ironically, had been the most apprehensive and anxious at the beginning—has been encouraged by his own malice and wishes to recklessly pursue his evil purpose further.
In Act 5, scene 1, Lady Macbeth is seen to sleepwalk as a result of her guilty conscience. Ironically, it was Macbeth who -- immediately after he killed Duncan -- feared that he would never be able to sleep peacefully again, and Lady Macbeth found him cowardly and weak. Now, it is she who cannot sleep peacefully. Further, after the murder of Duncan, Lady Macbeth had said, "A little water clears us of this deed. / How easy it is then!" (2.2.86-87). Now, however, she washes her hands over and over and over, believing that "All the perfumes of Arabia will not sweeten this little hand" (5.1.53-55). She once thought it would be easy to wipe the guilt away from her mind, just as she could wipe the blood off her hands; now, she feels, ironically, that the blood remains on her hands because the crime remains on her conscience. An additional irony is that, after Duncan's murder, she had told Macbeth that "These deeds must not be thought / After these ways; so it will make us mad" (2.2.45-46). Now, she is the mad one, hallucinating spots of blood still on her hands, reliving the night of the murder, conflating the murder of Duncan with Macbeth's murder of Banquo.