> During those 3 days all I really did was work and sleep.

This is why 3x12 is not workable for average families. If you have kids and want to see them, 3x12 only works if you start really really early, then get to bed early when the kids do too.

I enjoyed 4x10 when I did it, but there were some real problems with some employees trying to adapt. Anecdotally we were seeing a lot of people who would barely work until the 8 hour mark and then just zone out or socialize while they waited the clock out at the end of the day.

Which is all too bad for those of us who work well with longer days.

IME most people don't have more than 4 (maybe 5) hours of effective heavy cognitive / creative energy in a given day. Of course YMMV, and it's common to have periods of sustained output when deeply committed to and energized by something, but over the long haul, week in, week out, splitting the workday into roughly 8h sleep, 8h work, 8h everything else, with 2/7 days off plus many holidays and some weeks of paid vacation seems like a reasonable default to me -- for the traditional salaried role. If your preferences deviate much from that baseline, the entrepreneurial / consulting / fractional approach would probably be better aligned.