I'm much more productive with 10 hour work days. Get in the zone, write a shit-ton of code, and then get back to reality. Three or four 10-hour days sound a lot nicer than 5 6-hours.
Less commute time, more time to pursue hobbies and take trips.
I bet 4 * 9 hours days would have a negligible impact on worker productivity and reduce costs like heating, cooling, lighting, since offices would be closed an extra day.
A six hour workday would be 30 hours a week. Imagine that as three ten hour days instead. Wow, that would be fantastic.
I'm not a morning person, yet my job has me coming in before 8:30 so I can attend morning standups (ugh). Right after lunch though, I can easily get in the "zone" and do a ton of good work.