A 40 hour work week?

Part of me wonders where the standard of a 40 hour work week came from. My level of interest is not great enough to look it up on Wikipedia (the gold standard for researching such things, according to some of my students); I'm just curious. I know that it was associated with a number of labor reforms such as minimum wage and child labor laws. This is why I'm interested. Last Friday, July 4th, I spent a considerable part of the day in my office working on several research projects. And I felt that using this "holiday" to catch up on things was perfectly normal. This got me thinking about the amount of time that I spend working each week. Like many, I was warned during my PhD that it would usually be more than 40 hours a week. I think 60 was the most common response from assistant professors that I asked who were at Research 1 universities. I'm not at an R1 university, but I do think I get close to 50-55. And again, a few hours in the evening or on the weekend seems acceptable. I hate to think that I've given up the benefits that labor leaders fought so hard to provide me--it's just that I can't get it all done in 40 hours. I wonder how many hours are appropriate for assistant professors. Maybe I'll look that up on Wikipedia!