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There’s been a flurry of coverage praising Sheryl Sandberg, the chief operating officer of Facebook, for leaving the office every day at 5:30 p.m. to be with her kids. Apparently she’s been doing this for years but only recently “came out of the closet,” as it were.

What’s insane is that Sandberg felt the need to hide the fact, since there’s a century of research establishing the undeniable fact that working more than 40 hours per week actually decreases productivity.

In the early 1900s, Ford Motor Co. ran dozens of tests to seek the optimum hours for worker productivity. It discovered that the “sweet spot” is 40 hours a week — and that while adding another 20 hours provides a minor increase in productivity, that increase only lasts for three to four weeks, and then turns negative.

Anyone who’s spent time in a corporate environment knows that what was true of factory workers a hundred years ago is true of office workers today. People who put in a solid 40 hours a week get more done than those who regularly work 60 or more hours.

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The workaholics (and their profoundly misguided management) may think they’re accomplishing more than less fanatical workers, but in every case I’ve observed, the long hours result in work that must be scrapped or redone.

Accounting for Burnout

What’s more, people who consistently work long workweeks get burned out and inevitably start having personal problems that get in the way of getting things done.

I remember a guy in one company I worked for who used the number of divorces in his group as a measure of its productivity. Believe it or not, his top management reportedly considered this a valid metric. What’s ironic (but not surprising) is that the group accomplished next to nothing.

In fact, now that I think about it, that’s probably why he had to trot out such an absurd (and, let’s face it, evil) metric.

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Proponents of long workweeks often point to the even longer average workweeks in countries like Thailand, Korea and Pakistan — with the implication that the longer workweeks are creating a competitive advantage.

Europe’s Ban on 50-Hour Weeks

However, the facts don’t bear this out. In six of the top 10 most competitive countries in the world (Sweden, Finland, Germany, the Netherlands, Denmark and the U.K.), it’s illegal to demand more than a 48-hour workweek. You simply don’t see the 50-, 60- and 70-hour workweeks that have become de rigueur in some parts of the U.S. business world.

If U.S. managers were smart, they’d end this “If you don’t come in on Saturday, don’t bother coming to work on Sunday” idiocy. If you want employees (salaried or hourly) to get the most done — in the shortest amount of time and on a consistent basis — 40 hours a week is just about right.

In other words, nobody should be apologizing for leaving work at a reasonable hour like 5:30 p.m. In fact, people should be apologizing if they’re working too long each week — because it’s probably making the team less effective overall.

This post was originally published at Inc.com. James’ Sales Source (formerly Sales Machine on CBS) is the world’s most visited sales-oriented blog. His best posts, with many extras, are in his new book, How to Say It: Business to Business Selling. @Sales_Source

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