This advice only works if your side project and your day job aren't equally intense ie. if you're programing for your day job and your side project involves more programming, you will burn out. You can pretend that you relax with even more programming, but your body will disagree.
To make this work beyond a few months or a year as a programmer, you will either need to do an easier side project where ideally you're not doing the programming; or better yet you will need a very easy day job that doesn't use up the bulk of your daily creativity and problem solving stamina. Yes, it is a finite resource. The true ideal situation is being able to work on your personal projects during down time at your day job...
Also as someone else already mentioned, read your employment contract. Outside of places like California, the IP portions of that contract are draconian ie. your employer will own any idea that you generate whether it is on the job during office hours, or in your sleep or free time at home. This is even more important to keep in mind if you work remotely even just part of the time.