Read the contract. Does it say "terminate employment" or "quits". For most sign on bonuses you only lose it if you don't fulfill your end of the contract. If they fire you, you don't owe them anything. Tell HR you're feeling burnt out and need some time for family... In writing. Then be less of a perfect employee. If you're doing a PhD level job, mental health is a major part of productivity. You and the company have a vested interest in achieving an acceptable work / life balance. They obviously want you to stay. Be honest with yourself and find out what you can do to make it work.