The most important question, and the most cliché question of the decade! It has every possibility of making it into the Cliché Hall of Fame right alongside "paradigm", "synergistic", "stickiness" and "game changer".
The whole meaning of the question, "What keeps you up at night?" is to uncover the right--now issues that are a real concern. To be in a position to ask such a question requires you to be a trusted peer. It is NOT something to ask of a stranger or casual acquaintance. Unfortunately, too many people ask the question hoping for an answer to sell to -- not actually caring about the pain behind the answer.
Allow me to suggest that if you have a desire to know the answer to that question, spend the time it takes to be in a good relationship with the person before you ask.
Anyone can ask that question. Only someone I trust who knows me and my business will receive a truthful answer.