thanks for all your reply! It is very helpful!

I just had the phone interview. for answering this question, I mentioned that I need better work life balance. but he said that if I am looking for an easy job, this is not the job for me. So I tried to clarify and said that if there is a deadline to be met, I am willing to work overtime. I have the ability to do chanlenging work etc.

It looks that looking for better work life balance is not a good answer for hiring manager. They will suspect that I don't work hard.

I will summerize all you guys's ideas to come up with an answer that can satisfy the potential employer. Grneatly appreciated!