This long-ish article explains how we got it...

By 1914, emboldened by a dozen years of in-house research, Henry Ford famously took the radical step of doubling his workers' pay, and cut shifts in Ford plants from nine hours to eight. The National Association of Manufacturers criticized him bitterly for this -- though many of his competitors climbed on board in the next few years when they saw how Ford's business boomed as a result. In 1937, the 40-hour week was enshrined nationwide as part of the New Deal. By that point, there were a solid five decades of industrial research that proved, beyond a doubt, that if you wanted to keep your workers bright, healthy, productive, safe, and efficient over a sustained stretch of time, you kept them to no more than 40 hours a week and eight hours a day.

...how we lost it (once again, nerds ruin everything)...

The first is the emergence of Silicon Valley as an economic powerhouse in the late 1970s. Since WWII, the valley had attracted a unique breed of worker -- scientists and technologists who carried with them a singular passion for research and innovation. Asperger's Syndrome wasn't named and identified until 1994, but by the 1950s, the defense industries in California's Santa Clara Valley were already drawing in brilliant young men and women who fit the profile: single-minded, socially awkward, emotionally detached, and blessed (or cursed) with a singular, unique, laser-like focus on some particular area of obsessive interest. For these people, work wasn't just work; it was their life's passion, and they devoted every waking hour to it, usually to the exclusion of non-work relationships, exercise, sleep, food, and sometimes even personal care.

And then, in the early '80s, Tom Peters came along, and promoted the Silicon Valley work ethic to the rest of the country in the name of "excellence." He extolled tech giants like HP and Apple for the "passion" of their workers, and told old-industry employers that they could move into the new age by seeking out and rewarding that kind of passion in their employees, too. Though Peters didn't advocate this explicitly, it was implicitly understood that to "passionate" people, 40-hour weeks were old-fashioned and boring.

....and why it'd be good for everyone, employers included, to bring it back.

What these studies showed, over and over, was that industrial workers have eight good, reliable hours a day in them. On average, you get no more widgets out of a 10-hour day than you do out of an eight-hour day. Likewise, the overall output for the work week will be exactly the same at the end of six days as it would be after five days. So paying hourly workers to stick around once they've put in their weekly 40 is basically nothing more than a stupid and abusive way to burn up profits. Let 'em go home, rest up and come back on Monday. It's better for everybody.

In fact, research shows that knowledge workers actually have fewer good hours in a day than manual laborers do -- on average, about six hours, as opposed to eight. It sounds strange, but if you're a knowledge worker, the truth of this may become clear if you think about your own typical work day. Odds are good that you probably turn out five or six good, productive hours of hard mental work; and then spend the other two or three hours on the job in meetings, answering e-mail, making phone calls, and so on. You can stay longer if your boss asks; but after six hours, all he's really got left is a butt in a chair. Your brain has already clocked out and gone home.

The other thing about knowledge workers is that they're exquisitely sensitive to even minor sleep loss. Research by the US military has shown that losing just one hour of sleep per night for a week will cause a level of cognitive degradation equivalent to a .10 blood alcohol level. Worse: most people who've fallen into this state typically have no idea of just how impaired they are. It's only when you look at the dramatically lower quality of their output that it shows up. Robinson writes: "If they came to work that drunk, we'd fire them -- we'd rightly see them as a manifest risk to our enterprise, our data, our capital equipment, us, and themselves. But we don't think twice about making an equivalent level of sleep deprivation a condition of continued employment."