One of best ideas from agile methodologies is the balance of work and life. In the original XP book, Kent Beck laid down 40-hour week as one of the core practices of XP. However, in the second edition he started the term “Energized Work”, which is a bit vague. I guess if you want to be less controversial then choose more vague terms. Anyway, the idea is pretty much same that knowledge work such as developing software needs a relaxed mind. I have seen everywhere that when you are tired you make stupid mistakes and spend next day or days making up for it.
I read another interesting article Be smarter at work, slack off that reinforces same idea. It quotes Peter Capelli, a professor of management at Wharton, “You can turn a smart person into an idiot just by overworking him.”
Nevertheless, most of the real work still requires 50-60 hours a week and I have seen plenty of death-march projects and fear of loss of jobs to offshore has made things much worse.