I agree strongly with the idea that "40 hours" is just as made up as "15 hours" or whatever. We need actual data to draw any conclusions here. Similarly we haven't explored all the ways we can make people more productive either. Maybe if the employees who feel tired after six hours could take a long nap mid-day they'd be able to work 12 hours a day at a high level.

That said I'm on the work what you feel comfortable working and as long as goals are being met then it doesn't matter.