In the comments John Maxwell links again to a presentation claiming that folks who work more than about 40 hours a week don’t actually produce more:

Working more than 40 hours a week leads to decreased productivity. … >60 hour work week gives a small productivity book. The boost last 3 to 4 weeks and then turns negative. … Ford … [ran] dozens of experiments. As a result … he and his fellow industrialists lobbied Congress to pass 40 hour a week labor laws. Not because he was nice. He wanted to make the most money possible. … Performance for knowledge workers declines after 35 hours, not 40. … Past this they start becoming tired and making dumb decisions. (more)

The claim that Ford needed regulation to get his workers to work only 40 hours is clearly wrong. But the other claims are intriguing, and appeal to my contrarian tastes. These claims were also historically important:

During the first decades of the twentieth century, … a new cadre of social scientists began to offer evidence that long hours produced health-threatening, productivity-reducing fatigue. This line of reasoning, advanced in the court brief of Louis Brandeis and Josephine Goldmark, was crucial in the Supreme Court’s decision to support state regulation of women’s hours in Muller vs. Oregon. Goldmark’s book, Fatigue and Efficiency (1912) was a landmark. In addition, data relating to hours and output among British and American war workers during World War I helped convince some that long hours could be counterproductive. (more)

Now, the most productive people I know, including self-employed folks and those with a huge personal stake in their own productivity, tend to work tons of hours. Either these claims are just wrong about such folks, or they are right on average but don’t apply to the most productive folks, or these folks and their associaties consistently make a huge mistake (as did most of the working world before 1920). Which is it?

The presentation above cites this, which cites this, which cites books from 1894, 1908, 1909, 1913, 1926, and says:

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. .. Five-day weeks of eight-hour days maximize long-term output in every industry that has been studied over the past century.

This article quotes a 1980 article “Scheduled Overtime Effect on Construction Projects” as saying:

Where a work schedule of 60 or more hours per week is continued longer than about two months, the cumulative effect of decreased productivity will cause a delay in the completion date beyond that which could have been realized with the same crew size on a 40-hour week.

I couldn’t find that source, but I found a 2001 review article:

Based on the foregoing overview of available studies it is evident that only a few are based on original data. Moreover, less than reliable data have been published and republished over and over giving a false appearance of originality. Finally, data are available for a limited number of trades only. Figure 16 compares the reported efficiency from various studies for the 50-hour, 60-hour and 70-hour work weeks with the majority based on 10-hour workdays and an overtime schedule of four consecutive weeks. (more)

That figure 16 estimates a max total productivity over four weeks at 60 hours per week. But the study it cites that looked longest, found that by sixteen weeks median per hour productivity had fallen by 30%, 50% and 62% for 50, 60, and 84 hour work weeks. (Though for that source “The origin of the data and the work environment are unknown.”) So yes, the basic claims above do weakly check out, at least for the construction industry. But basic questions still remain: How solid is the data here, does this apply to all industries, and does it apply to our most productive workers?