For the last 100 years, every productivity study in every industry has come to the same conclusion: After about 40 hours in a week, the quality of your work starts to degrade,” she writes. “You make mistakes. That’s why working 60 hours may not save you time or money: You’ll spend too much of that time fixing the mistakes you shouldn’t have made in the meantime. That’s why software companies that limit work to 35 hours a week need to employ fewer QA engineers: There isn’t as much mess to clean up.