I have a four-day, 32-hour work week, and I am less productive, but that was the plan up front and I'm very okay with that (and luckily, so is my management).

I wanted more personal time, and I also needed more time to take care of my physical and mental health with appointments -- which all of my providers schedule during the 9-AM-to-5-PM period anyway -- and simple rest and relaxation. If I didn't have this schedule, there's a good chance I wouldn't still be at this current job or would have burned out earlier in general, and my contributions are valued, so it works out well for all parties involved.

But I don't think you have to justify it so concretely or morally -- you can just want to do a day's less formal work, or want to work toward a society with less work and more leisure time! The 40-hour work week is arbitrary, anyway.