Macbeth believes he heard a voice crying:
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,(50)
Chief nourisher in life's feast— - See more at: http://www.enotes.com/topics/macbeth/etext/act-ii#etext-act-ii-act-ii-scene-ii
What this imaginary voice is suggesting by "Macbeth doth murder sleep" is that Macbeth's crime is especially heinous because he killed a man while his victim was sound asleep. Since the murdered man was totally defenseless, he will serve as an example to everyone--and especially to Macbeth himself--that sleeping is not safe. It should be a state of peace, comfort and security, but Macbeth's example has made it a state of extreme danger. The voice continues:
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.”( - See more at: http://www.enotes.com/topics/macbeth/etext/act-ii#etext-act-ii-act-ii-scene-ii
Macbeth has seen how vulnerable a king can be when he is sound asleep, and now he proposes to become the king himself. He is accepting the danger that goes with the position. Therefore he will be afraid to go to sleep and will suffer from insomnia for the rest of his life. This lack of sleep will drive him half insane and cause him to behave more and more erratically and tyrannically. He can't sleep and he can't trust anybody. It wouldn't help him any to be guarded by attendants while he tried to go to sleep. Duncan had two grooms supposedly guarding him, but they were useless.
Macbeth realizes he is setting a bad example that others might follow. Earlier he says to himself:
But in these cases
We still have judgement here, that we but teach
Bloody instructions, which being taught return
To plague the inventor. This even-handed justice(10)
Commends the ingredients of our poison'd chalice
To our own lips. - See more at: http://www.enotes.com/topics/macbeth/etext/act-i#sthash.QjWyKeH6.dpuf
If a good king like Duncan can't safely close his eyes and go to sleep, then how could a murderer and usurper expect to do so?
Macbeth appears to be developing a split personality even at this early stage, just after he has committed the murder. In saying that Glamis has murdered sleep and therefore Cawdor shall sleep no more, the voice is suggesting that one half of Macbeth represented by Glamis feels shame, guilt, and pity, while the other half represented by his new identity as Thane of Cawdor will be punished with lifelong insomnia for making Glamis do the deed. When Macbeth inherited the title of Cawdor, he must have inherited the treacherous nature exhibited by the former thane who was executed on Duncan's orders.
Macbeth is mainly suffering from guilt. Guilt, we are told by psychologists, is based on fear of exposure and punishment. It is very hard to go to sleep when we are frightened--although that is a time when we would very much like to be able to fall asleep.