I have never been a good worrier. And I've always been an excellent sleeper. So, really, nothing keeps me up at night worrying. I am generally asleep within 3 minutes of turning out the lights.
That's not to say I haven't had times that gave me pause. I sure have. But, thinking them through and developing a plan is the way I deal. Hand wringing has never done it for me. And, honestly, old age gives me a great perspective. This, too, shall pass. Nothing to waste a worry on.
I have been really tense at times about certain situations. I worry before my annual checkup. I really worry if I have to go to the dentist. But those worries have a definite end date.
The secret, in my book, is to make sure you have explored worst case scenario and developed a plan for same. Once you come to terms with the worst that can happen, there really isn't anything left to worry about.
I have, however, successfully used this question often in job interviews. It's generally produces great and useful info about the company or the person. What about this company(or your job) keeps you up at night worrying?