I’m all for working long hours and I remain convinced that I am contributing to the work/life balance argument by not having a life. But:

In the early 1900s, Ford Motor ran dozens of tests to discover the optimum work hours for worker productivity. They 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.

Stop Working More Than 40 Hours a Week – Geoffrey James, Inc (24 April 2012)

That’s the crux of it, really, but read the full piece for a little more of the history of the 40-hour figure and how it’s applying to office workers as well as Ford’s factory ones.