Completely agree. And the fact that the breaking point is a nice round 40, which just happens to correspond with our culture's 5-day work week and typical 8 hour day is a little too convenient to believe.
I'd believe that's true, but I'd also believe there's reason to suggest that what holds for factory workers doesn't hold for information economy workers. (Although anecdotally, I find that I'm WAY more productive when working closer to 6-hour work days than 8-hour days.)
I'm glad someone else has noticed this. The fact that I still end up feeling burned out if I constantly work 8 hour days has made me feel very lazy. It's weird, I feel like I can work at top speed all the time if I work 6 hour days, but if I try to maintain that same pace with longer days then I hit a wall. In effect, I need to pace myself to work 8 hour days.
The only time I feel like I need a day longer than 6 hours is when I'm in meetings all day. Meetings and working seem to use two different kinds of energy. So I could meet for 3 hours about something and work for 6 hours just fine, but I'd be wasting my time if I was attending 3 hours of meetings each day.
The thought above pretty much reinforces my belief that the hours we work have nothing to do with productivity, and everything to do with an illusion of fairness. My boss spends more than half his day in meetings. It probably wouldn't look good if I was working 3 hours less than him each day just because I don't have as many meetings to attend. The issue is even trickier when I consider the fact that many of my coworkers need less concentration to perform their roles, and would be perfectly fine working 5x10s by their own admission.
I always thought: we work 40 hours a week because we work 8 hours a day; and we work 8 hours a day because Robert Owen's mantra "Eight hours' labour, Eight hours' recreation, Eight hours' rest" caught on.
[1]: Unfortunately I don't have them handy, and they had some clear limitations. If I'm remembering correctly, some were about relatively unskilled work and none were about programming, where measuring productivity is an open problem.