And if it's not been rigorously tested, then there doesn't seem to be any strong case to assume that the ideal number for office work is higher, rather than lower, than 40.

I'm also wondering how commute time (and method of commute) factors into this. Does 30 minutes of physically strenuous cycling before/after work have a positive or negative impact on office workers? Does the same time spent in a car navigating stop'n'go traffic have a different effect? Both involve some element of mental effort, but both are quite different from the rest of the days tasks. Certainly in my experience one helped my mental state and one didn't.