We'd probably be better off overall with even less, like a 30-32 hr work week for most jobs. (Assuming, of course, that the wages are adjusted so 30-32 hrs/week pays a living wage.) 40 hours a week is already pushing the limits of how long humans can maintain productivity in many environments, and when you push past that for an extended period of time, overall productivity actually drops, due to people slowing down from exhaustion, making more mistakes, etc.

I haven't seen studies on <40 hour weeks, but I'd bet that there are cases where dropping down to 30 or 32 would actually increase total productivity, because people are more focused, more rested, and less resentful because their jobs aren't eating their lives.