Macbeth's plan to kill King Duncan throws the Elizabethan universe out of order. God had placed the king as head of Scotland: disturbing that order causes unnatural occurrences. There is stormy weather; there is darkness during the day; there are horses eating each other...An extension of this is Banquo unable to sleep.
There’s husbandry in heaven;
Their candles are all out. Take thee that too.
A heavy summons lies like lead upon me,
And yet I would not sleep.