It is stating the obvious to write that Shakespeare was a genius, but every once in a while, that fact is brought home to me with such clarity that I have to do it. As writers, we know about themes and how they enrich a story. In MACBETH, the theme of sleep is handled brilliantly.
At the outset, Macbeths have no idea that murder sticks with a person. They think (although he is less convinced of it than she is) that once the king is dead, everything will be fine. But once it is done, they are haunted by their action. The murder eventually brings about their doom, but in the short term, it manifests most clearly in sleep. He becomes unable to sleep at all. She does, but we see the turmoil that occurs when she lets her subconscious take over.
I don't think mystery writers take enough note of such things. Not being murderers ourselves, we might assume that they go blithely on their way, unbothered by their crimes. While that may be true of psychopaths and sociopaths, an ordinary person who kills will suffer afterward. Shakespeare's demonstration of that, while not the only one, is a good one. For most of us, sleep restores, puts our troubles into perspective, and allows us to begin anew. For the guilty, I would guess that sleep is evasive and likely to further unravel that "sleeve of care".