Side projects are one of the biggest battery booster you can have.

For me it's common to get burnt out with fixing a bug or refactoring some pretty old logic, and working in side projects is a nice break to recharge the batteries without necessarily getting your mind totally out of programming. It doesn't matter if they are yours our someone else's, they can do wonders.

(Also, it's good to learn different stuff from what you are used to, like frontend developer working on some back-end logic in a side project)