Actually I think martial arts, when done at a high level, is more mental than physical. One of the reasons I got out of it years ago is that I didn't have the mental energy at the end of many days to focus for 2 hours after a long commute.

You're right that we shouldn't take our jobs too seriously, but it's not always a question of being too intense or serious. There are plenty of times someone pushes a button or upsets us over something that we wouldn't care about most days. Physically stressing the body can bleed off the mental stress, sometimes just because it distracts you.

These days I try to go hard every night, and I find that the days when I'm not 100%, I'll actually get into more of a zone and work harder. It's a nice release after typing for hours or trying to focus on writing.