This absolutely. I work long hours coding on my PhD (because I love it), then I come home and work the rest of the hours on side projects. Over the course of a

This absolutely. I work long hours coding on my PhD (because I love it), then I come home and work the rest of the hours on side projects. Over the course of a few months my productivity wanes a lot - I can feel the burnout creeping on. Over the last two years I've figured out that taking an hour out to go for a walk every day makes a huge positive difference. Sometimes, like this week, I take a week off and just do whatever I want as long as it's not my PhD. Afterwards I'm refreshed and ready to hit it hard again.