40 hour work week was a reaction on the employers with too much power. They pushed their employers to work beyond their physical health limits. As a rule, a more balanced work structure was introduced that protected both partys on the long run.

There have been a lot of research about most productive work rythm. But it differs a lot depending on the kind of work. But 40 hours is a general avarage for most people.

The end conclusion is, depending on the work and your personal context, you have to find a balanced life where.