Banquo can't sleep because he is "haunted" by unpleasant thoughts, thoughts that he calls "cursed."
This starts after hearing the witches' prophecy, so we can assume those are some of the thoughts haunting him.
He is also concerned with others being up and about (Macbeth in particular); at this point it starts to become symbolic. There is a natural order to things in Shakespeare's world, and if it is upset, it means things are not right. If people are up at night, they are doing unnatural, unethical things, as, in fact, Macbeth is.