Productivity is a finite resource

When you start depleting it, eventually you will run out. At that point, the only way to be productive again is to not try being productive :)

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Why Crunch Modes Doesn't Work: Six Lessons
Executive Summary. When used long-term, Crunch Mode slows development and creates more bugs when compared with 40-hour weeks. More than a century of studies show that long-term useful worker output is…