What does Macbeth shall sleep no more mean?

MACBETH

Methought I heard a voice cry 'Sleep no more!
Macbeth does
murder sleep', the innocent sleep,
Sleep that knits up the ravell'd sleeve of
care,
The death of each day's life, sore labour's bath,
Balm of
hurt minds, great nature's second course,
Chief nourisher in life's feast,--

LADY MACBETH

What do you mean?

MACBETH

Still it cried 'Sleep no more!' to all the
house:
'Glamis hath murder'd sleep, and therefore Cawdor
Shall
sleep no more; Macbeth shall sleep no more.'

LADY MACBETH

Who was it that thus cried? Why, worthy thane,
You do
unbend your noble strength, to think
So brainsickly of things. Go get some
water,
And wash this filthy witness from your hand.
Why did you
bring these daggers from the place?
They must lie there: go carry them; and
smear
The sleepy grooms with blood.

Macbeth realizes what he has
done.  The dichotomy that arises here is that one murdered man laughs when murdered and
the other cries, "Sleep no more".  Macbeth's own mentality is finally at question.  He,
nor is anyone else, aware that he is in the process of going mad.  The laugh is symbolic
of his upcoming madness.  The "Sleep no more" refers to Macbeth's inability to truly
rest now that he has committed murder.  His conscious will not allow him the rest. 
Macbeth is no longer innocent as the men who were murdered in their sleep and,
therefore, can no longer sleep because only the innocent can sleep soundly. It is
Macbeth's fault that he is in this predicament and he is just realizing the consequences
of his actions.  Fortunately, he does have enough sense to realize that his life is
about to change dramatically.