why does Banquo not sleep?

Banquo cannot sleep because his murder has separated him from God and he realizes it when he cannot say Amen. He no longer has the ability to feel comfortable in his life; he cannot sleep because he has condemned himself, through his murder of Duncan, to a life of consideration of what a horrible person he is.

Banquo didn't murder Duncan. Banquo could not sleep because Macbeth had altered the delicate order of the Elizabethan universe by murdering Duncan. Banquo felt all strange because something evil was about to happen, something that would make God angry. Thus, that evening there were storms, crazy horses....

Oops - sorry! I was in such a hurry, I didn't even think of Banquo. Banquo could not sleep early in Act 11 because he has to think about what the witches have said. He encounters Macbeth and they talk about "what hath chanced." He senses that something is about to happen - and of course, after he leaves, Macbeth and his wife continue with their evil plan.