It is night in Macbeth's castle of Dunsinane. A doctor and a gentlewoman wait. The gentlewoman called the doctor because she has seen Lady Macbeth sleepwalking the last few nights, but she refuses to say what Lady Macbeth says or does.
When he killed Duncan, Macbeth thought he heard a voice say he had murdered sleep. Well, he did: Lady Macbeth's sleep.
Lady Macbeth enters, holding a candle, but asleep. Lady Macbeth keeps rubbing her hands as if to wash them while saying "out, damned spot" (5.1.30). Then Lady Macbeth seems to relive her attempt to convince Macbeth to kill Duncan, concluding with the words: "Yet who would have thought the old man to have had so much blood in him" (5.1.33-34)?
The horrified doctor and gentlewoman watch as Lady Macbeth then relives conversations with Macbeth after the murder of Banquo and hears an imaginary knocking and rushes off to bed. The doctor says the disease is beyond his power to cure, and that "unnatural deeds do breed unnatural troubles" (5.1.61-62). He also says he dares not speak about what he's just witnessed.
Lady Macbeth's guilt makes it impossible for her to hide the horrors that she and Macbeth have committed. Her conscience is rebelling against the unnatural fiend that ambition has turned her into.