“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.”

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Stop Working More Than 40 Hours a Week | Inc.com | 4.24.12

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