Why does Macbeth say "Glamis hath murdered sleep, and therefore Cawdor / Shall sleep no more. Macbeth shall sleep no more"?
The lines that Macbeth speaks to his wife about sleep are among the most beautiful Shakespeare ever wrote. Here they are in full:
Me thought I heard a voice cry “Sleep no more!
Macbeth doth Murder sleep”—the innocent sleep,
Sleep that knits up the ravell'd sleave of care,
The death of each day's life, sore labor's bath,
Balm of hurt minds, great nature's second course,
Chief nourisher in life's feast—
What do you mean?
Still it cried, “Sleep no more!” to all the house;
“Glamis hath murdered sleep, and therefore Cawdor
Shall sleep no more. Macbeth shall sleep no more.”
The whole play, like all Shakespeare's plays, is written in poetry recited by the various characters. Shakespeare's poetry, it should go without saying, is what is best in Shakespeare. Shakespeare's poetry is Shakespeare. Here Macbeth is quoting a voice he thought he heard. He says it cried "Sleep no more!" to all the house. That seems intended to answer the question of why he doesn't murder Malcolm and Donalbain while he has an opportunity he will never have again. Malcolm is the heir apparent. Duncan has proclaimed his elder son Prince of Cumberland and his immediate successor to the throne. Macbeth can hardly expect to become king of Scotland just by murdering Duncan, but if he intended to murder Malcolm and his brother he was afraid the voice crying "Sleep no more!" would wake up the entire household and he would be caught with blood on his hands and clothing and two bloody daggers in his hands. (He may have intended to use the daggers to kill Duncan's two sons.)
The voice that threatens to wake the house says,
"Glamis hath murdered sleep, and therefore Cawdor
Shall sleep no more. Macbeth shall sleep no more."
What this means in plain language is that since Macbeth, who is both Thane of Glamis and Thane of Cawdor, has murdered the king in his sleep, he knows how easily this can be done. He has the bloody scene permanently imprinted in his mind. Every time he closes his eyes he will see Duncan's murdered body. When Macbeth becomes king himself he will never be able to sleep soundly because he will always be afraid that someone might be planning to sneak up on him in the middle of the night and cut his throat. He says earlier in a soliloquy in Act 1, Scene 7:
But in these cases
We still have judgment here, that we but teach
Bloody instructions, which, being taught, return
To plague th' inventor. This evenhanded justice
Commends the ingredience of our poisoned chalice
To our own lips.
We don't have to wait for "the life to come" for our punishment for our misdeeds. We set examples which others can and will follow. If Macbeth murders Duncan, then he can expect somebody else to murder Macbeth. If people can become kings by committing murders, then others will get the same ideas. In our day these are sometimes called "copycat murders." Macbeth is simply saying that he will never be able to sleep soundly again for the rest of his life because he can imagine the same horrible thing happening to him in his sleep that happened to King Duncan.
The voice calls Macbeth "Glamis" and "Cawdor," suggesting that Macbeth has too many titles and can't handle them all. He doesn't know who he really is. If he adds the title of "King" to his other titles he will be even more confused. There is a part of him that is good and doesn't want to murder anybody, but whoever tries to murder him in his sleep will murder the good Macbeth along with the bad one. If Glamis gets to sleep, then maybe Cawdor will lie awake; and if Cawdor manages to get to sleep, then maybe Glamis will wake up! It is at this point that Macbeth seems to be developing a split personality. He is obviously acting crazy in this scene with his wife, and he will be acting more and more crazily as the play progresses. He is already having hallucinations, and he will have worse ones. Macbeth is an ideal candidate for schizophrenia because he actually does have several different identities. He is Thane of Glamis, Thane of Cawdor, and King of Scotland. As Thane of Cawdor he seems to have inherited both the former thane's title and his evil, treasonous nature.
Right away the prophecy of the voice that cried "Sleep no more!" seems to start being fulfilled. Macbeth wants to wash his hands and go to bed. He wants to be asleep when Duncan's body is discovered. But a knocking at the gate begins in that same scene, and Macbeth is finally forced to go down and see why nobody is responding. Everybody is able to sleep but Macbeth. As a result he has to be present when Macduff discovers Duncan's body, and Macbeth never gets a wink of sleep for the whole night.