The Pomodoro technique can definitely be a lifesaver in those slump days. I've tried a few Pomodoro apps on Linux myself. They each have their unique offerings, and it genuinely comes down to your personal preferences.
An additional tool I found useful while managing my productivity is this
performance tracking tool from EmpMonitor. Along with the Pomodoro technique, tracking my performance with this has given me insights into my most productive hours and areas to improve.
