Shakespeare uses the motif of night and dark in Macbeth to establish mood. Mood in turn is very effective for transmitting theme. We grow aware of the dark side of ambition, which "conspires with unholy forces to commit evil deeds which, in their turn, generate fear, guilt and still more horrible crimes" (enotes, themes).
The mood of the play is dreary at times and downright malevolent at others. The motif of darkness is established in the first scene of Act I with the presence of the witches and references to “thunder, lightning” and the “hurly-burly” of battle (enotes etext pdf, p. 8). The heath is also dark. Banquo describes the witches as “instruments of darkness” (p. 15)
Throughout the story, Macbeth darkens. The motif of black and dark is like an outward representation of his soul. It is no coincidence that Lady Macbeth calls upon the darkness right before her husband enters.
Come, thick night,
And pall thee in the dunnest smoke of hell,
That my keen knife see not the wound it makes,
Nor heaven peep through the blanket of the dark
To cry, “Hold, hold!”(55) (Act 1, Scene 5, p. 20)
Lady Macbeth darkens the mood, and Macbeth steps into it. Until now he has shown some signs of darkness, especially when he announces “stars, hide your fires;/Let not light see my black and deep desires” (p. 18). He continues the spiral into the dark side when he kills Duncan.
Me thought I heard a voice cry “Sleep no more!
Macbeth doth Murder sleep” (p. 30)
Macbeth goes on a killing spree then, killing Banquo and Macduff’s family. His descent into darkness has been paralleled by the motif of dark and night repeatedly mentioned by different characters. By the end, we are well aware of the dangers of ambition.
For the entire etext, read here: http://www.enotes.com/macbeth-text
For more on themes, read here: http://www.enotes.com/macbeth/themes