Relaxation is nice, we tell ourselves. Downtime to rest and unwind is a worthwhile endeavor as it combats stress and puts the mind and body at ease.
Unless you've got the thing you must do. The skill you want to practice. The weight you want to lose. The book you want to finish (reading or writing). The person you need to talk to. The idea you need share. The difference you want to make.
In these cases, relaxing is nothing more than procrastination. You think you're weary, but you are just prolonging your weariness. There is no better rest than that which comes after the work is truly finished.
Finish the work, then relax. In that order, every time.