In 1908 - almost a century ago - industrial efficiency pioneer Ernst Abbe published in Gessamelte Abhandlungen his conclusions that a reduction in daily work hours from nine to eight resulted in an increase in total daily output. (Nor was he the first to notice this. William Mather had adopted an eight-hour day at the Salford Iron Works in 1893.)
... When Henry Ford famously adopted a 40-hour workweek in 1926, he was bitterly criticized by members of the National Association of Manufacturers. But his experiments, which he'd been conducting for at least 12 years, showed him clearly that cutting the workday from ten hours to eight hours — and the workweek from six days to five days — increased total worker output and reduced production cost. Ford spoke glowingly of the social benefits of a shorter workweek, couched firmly in terms of how increased time for consumption was good for everyone. But the core of his argument was that reduced shift length meant more output.
I have found many studies, conducted by businesses, universities, industry associations and the military, that support the basic notion that, for most people, eight hours a day, five days per week, is the best sustainable long-term balance point between output and exhaustion. Throughout the 30s, 40s, and 50s, these studies were apparently conducted by the hundreds; and by the 1960s, the benefits of the 40-hour week were accepted almost beyond question in corporate America. In 1962, the Chamber of Commerce even published a pamphlet extolling the productivity gains of reduced hours. But, somehow, Silicon Valley didn't get the memo.