What is "somnambulism" in Macbeth?
Lady Macbeth's sleepwalking has become so chronic that a doctor has been summoned but, as my colleague has said, her problem is not physical, it is spiritual (this is the doctor's assessment). We know her to be wracked with guilt over her part in what she's done. Although she was the first to attempt to repress the feelings of guilt Macbeth had, she is incapable of controlling herself by the end of the play. Her sleepwalking reveals what she cannot come to turns with in her waking life. In this she operates as something as a foil to Macbeth. While Macbeth acknowledges his guilt immediately, she refuses to think about it. Macbeth is later able to rationalize his acts (sort of) and continue living the lifestyle he has chosen, but she cannot. Instead, she becomes more and more consumed by guilt until she can no longer live with what she's done.
To an elizabethan mind, her sleepwalking would seem unnatural, much like the deed that was done.
Somnambulism is sleepwalking. Sleepwalking occurs in Act V where the doctor and Lady Macbeth's nurse watch her as she sleep walks. She is so disturbed by the murder that she and her husband have committed that it haunts her mind and won't let her sleep peacefully. She says some very incriminating things while she is sleepwalking.