Is there any reason to, in principle, oppose a 4 day work week?
Why not pursue a 4 day work week?
Indeed.
Training for eight hours seven days a week does not make you a stronger athlete. Quality training, not quantity, is the key, including quality recovery time in between. When it comes to brain & body, longer work days and/or longer work weeks does not mean better quality work or more efficient work tempo.
In fact, workplaces that have shortened their/trialed shorter work week and/our hours have largely found the opposite true: well rested, less stressed and happy workers = more efficient, more productive, more innovative workers and results.
While shortening daily hours and/or work week has increased salary costs as more workers have been needed to do the same job, the savings in terms of fewer sick days along with the actual increased productivity ("more for less") have been greater. For example, Toyota in Sweden switched from a 40-hour to 30-hour work week in 2003 and is
happy with the increased profits and productivity. Their mechanics actually get more work done in 30 hours a week than before. What they did was to lengthen opening hours from 9 to 12 and split it in two 6-hour shifts, with the result of happier workers (plus more jobs --> happier taxpayers), happier customers and happier shareholders -- a triple win-win-win for all.
It's this kind of smart things that make sense. A five-day 40-hour work week certainly isn't a God-given the most perfect thing to manage our economy and lives and no reason to treat it like a Golden Rule-never-to-be-broken-or-end-of-world.