(**Note that I use the Open Source Shakespeare site for lines. When I include line numbers, I refer to TLNs or "through line numbers" which numbers lines from the beginning of the play to the end.)
The lines that "billdelaney" refers to in his answer above appear in Macbeth’s speech in Act 2 scene 2 lines 694-699. Macbeth's speech reflects his initial horror at his murder of King Duncan.
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,—
Here he first refers to sleep as “knit[ting] up the ravell’d sleeve of care.” Imagine a day’s cares or troubles as unravelling a sweater sleeve--pull on the yarn, and the sleeve shrinks as the knitting is undone. With sleep, the sleeve is restored or knitted up again.
Next Macbeth calls sleep “the death of each day’s life.” Sleep is a commonly used as a metaphor for death in literature and in life. Sleep appears often as a metaphor for death in the play.
Lady Macbeth compares sleep and death a few lines after Macbeth’s speech above. As she chides Macbeth for fearing to go back to the murder scene to plant the daggers, she says: “...the sleeping and the dead/Are but as pictures...”
In the next scene, as Macduff wakes the household to the news of the dead King, he cries, “Shake off this downy sleep, death's counterfeit,/And look on death itself!”
In the last lines of Macbeth’s speech, Macbeth then compares sleep to a soothing bath to ease a sore body after a day of labor. Sleep is a balm or salve applied to hurts as in “balm of hurt minds.” Finally, sleep is “nature’s second course/chief nourisher in life’s feast.” The images are of sleep as a bath, a balm, and a nourishing feast.
But Macbeth has murdered this innocent sleep.
Characters who cannot sleep cannot partake of this nourishing feast and suffer from their lack of or inability to sleep. First there’s the mention of the sailor cursed by the witch in Act 1 scene 3 lines 117-120:
Sleep shall neither night nor day/
Hang upon his pent-house lid;
Weary se'nnights nine times nine
Shall he dwindle, peak and pine:
Another example of a character unable to sleep is in Act 3, scene 2, where Macbeth talks of terrible dreams that haunt him, and suggests that Duncan “sleeps well” (another comparison of sleep to death):
...the affliction of these terrible dreams
That shake us nightly: better be with the dead,
Whom we, to gain our peace, have sent to peace,
Than on the torture of the mind to lie
In restless ecstasy. Duncan is in his grave;
After life's fitful fever he sleeps well;
In Act 5 scene 1, the Gentlewoman and the Doctor observe Lady Macbeth sleepwalking, showing that her mind is troubled. She, like the poor cursed sailor, is unable to partake of the nourishment of sleep that Macbeth refers to.
For Lady Macbeth and Macbeth, the murder of Duncan has murdered their ability to sleep “the innocent sleep,” the sleep that is a nourisher, a balm, a bath, and a hand that "knits up the ravell'd sleeve of care."
Act 2, Scene 2, line 47 and following contain one of Shakespeare's most beautiful passages, beginning with:
"Methought I heard a voice cry 'Sleep no more!' This soliloquy should provide all the sleep imagery you need. The six lines are full of metaphors for sleep.