In Act 5, Scene 1, Why does Lady Macbeth "take forth paper, fold it, write upon't, read it, afterwards seal it, and again return to bed"?
This action is simply apart of Lady Macbeth's sleepwalking. She has become so disturbed and full of guilt because of the knowledge that she and her husband murdered the King that she can no longer sleep. Instead, she nightly gets out of bed, writes on paper, "washes" her hands, and talks about Banquo's murder.
Lady Macbeth: Wash your hands, put on your nightgown; look not so pale.--I tell you yet again, Banquo's buried; he cannot come out on's grave. (5.1)
In this scene, her lady-in-waiting and her Doctor are watching her. The Doctor is there to try and help Lady Macbeth, but, in the end of the scene, he comments that her distress is a sickness of the mind and that he does not have the medicine for it: "More needs she the divine than the physician./God, God forgive us all!" (5.1).
It was in a letter Lady Macbeth received in Act 1, scene 5, from her husband that she became aware of the witches' prophecies for Macbeth. As soon as she read his account of his encounter with the witches, she became determined that Macbeth would become king. It was then that she summoned the spirits to "...unsex me here,/And fill me from the crown to the toe top-full/Of direst cruelty! make thick my blood;..." (1.5). She wants to make her blood "thick" so that she'll have whatever it takes to make sure her husband becomes king. The play comes full-circle in Act 5, scene 1, as Lady Macbeth continues to fold and unfold a letter in her state of madness. The spirits did indeed give her the will to plan a murder, but she lacked the cruelty to stave off the guilt.
Lady Macbeth's letter-writing is analogous to her emotional state. Lady Macbeth is seen to "...unlock her closet, take forth paper, fold it, write upon't, read it, afterwards seal it" and presumably confess her feelings into the letter she writes. This is analogous with Lady Macbeth unlocking her heart and pouring forth fears and anxieties she cannot confide to anyone during waking hours, especially as even her husband is away ("Since his majesty went into the field, I have seen her rise from her bed,..."). It is only during nighttime that her pent-up emotions find expression.
She always seals up her letters, then returns to bed as if nothing happened. This is just what Lady Macbeth has to do psychologically--after briefly revealing her dark secrets, she has to promptly lock them back deep inside her heart again lest they be known to the others.
The correspondence between Lady Macbeth's behaviour and her inner feelings is again seen in her act of washing. While outwardly she rubs her hand, it is in fact analogous with her desire to cleanse her mind of dark thoughts and emotions.
Gentlewoman: It is an accustomed action with her, to seem thus washing her hands: I have known her continue in this a quarter of an hour. 5.1
In this scene, a Lady in Waiting reports that before the scene began she had witnessed a psychotic, sleep walking Lady Macbeth take a paper, write on it, read it, seal it, and then return to bed. It seems likely that Lady Macbeth was writing statements on the paper that declare her role and guilt over the killings of Duncan and Banquo. Later in the scene, in front of the Doctor and the Lady in waiting, she makes additional comments that confirm her guilt.