Scene 1, lines 6-20 : read ahead to find the probable cause of banquo's insomnia. what is it?

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.