I doubt the breaking point is 40, but 40 seems close. I remember looking through studies on the matter[1] and seeing numbers floating around between 35–45 hours/week. It's hard to generalize because productivity is hard to measure and behaves differently in different cultures and professions, but I am willing to believe that the breaking point for most people is within 10—15% of 40 hours/week. Then again, I wouldn't be surprised if it was a bit lower...

[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.