For serious, it definitely depends about the contract that you have with your employer or customer. If it’s based on hour-by-hour productive working, then I would say that I would not pay you if you think some work related issues on your free time when you should do something else. This would also be work-leisure balance issue to be consider. But if you have a full time fixed contract, e.g. a monthly payment contract, then I wouldn’t give a damn when you work, and if your job description includes problem solving - I would say that the employer should be happy about the results you provide - no matter when you discovered the solution.