> You can't leave your brain at work if your job is mostly mental work.

Trust me, you can. I simply ignore emails after 6pm. I will look at them first thing at 8am the next day, but not after 6. If someone presses you on it and they won't take "I have a life" for an answer, just lie and say you didn't see it or that you were at your kid's baseball game. I suspect that the frequency of someone's boss actually getting pissed that they did not respond right away to a 9pm email to be extremely low. In the unlikely occurrence that you actually work in an environment like this, I suggest polishing up the resume, because there are plenty of jobs out there that do not require your brain 24/7.