How do you interpret Macbeth's closing line in Act 2, scene 2: "Wake Duncan with thy knocking! I would thou couldst!" (2.2.72)?

In Shakespeare's Macbeth, after he
assassinates Duncan, Macbeth hears a voice that tells him he has murdered sleep.  In
delivering Duncan to the big sleep, as they say, to figurative sleep, to death, Macbeth
himself will sleep no more. 

Macbeth panics after he kills
Duncan, feeling that he will never be able to get Duncan's blood off his own hands, and
that if he were to wash his hands in the ocean, there is so much blood that it would
turn the sea red.  He is panicked and a bit out of control.  When he hears the knocking
on the door (Macduff, soon to be Macbeth's nemesis, has arrived, though Macbeth doesn't
know who is knocking), he speaks the lines you ask
about. 

For the moment, Macbeth regrets killing Duncan, and
wishes he were still alive so something like knocking could wake him.  The first line is
self-directed sarcasm:  wake Duncan with your knocking!  The second line is his
expression of sorrow and regret:  I wish knocking could wake
him. 

What Duncan will do forever, Macbeth doesn't do that
night--sleep.  And his sorrow and regret is short-lived.  Moments later he kills the
grooms, presumably to shut them up, then devises a quick, complex argument to explain
away his doing so. 

Later, however, Macbeth will wish he
could have the peace Duncan has in death.  And Duncan has something else Macbeth no
longer has--the ability to sleep.  Macbeth's closing lines in this scene contribute to
and further the theme of insomnia in the play.