I'd think the "What keeps you up at night" is in reference to what difficult problems are you encountering at work. I'd asked my previous CTO this question (whe

I'd think the "What keeps you up at night" is in reference to what difficult problems are you encountering at work. I'd asked my previous CTO this question (when I was working for him already) and his response was "Finding the best people to hire" which I think is a fair response. He was trying to get me to refer people to join.