I agree. A 4 day work week has been borderline life changing for me.

I think 3.5 days might be the sweet spot. The issue is I very much enjoy my work but I find my productivity is the same or sometimes even better compared to when I worked a 5 day/40 hour week. I just use my hours better.

I should also say my managers have been great about massively reducing my meeting time in response to our company going 4 days a week. I went from 2-3 hours worth of meetings a day to an average of about 20 minutes (the morning standup).

Finally, one other thing I think is worth noting is that I tend to spend a portion of my extra day off studying various compsci/programming problems which have often benefited my company. I think they are essentially getting 0.5+ days worth of skunkworks research. In previous jobs where I would work upwards of 80 hours on fairly conservative work I didn't have the motivation to improve in my off hours.

Economist John Maynard Keynes discussed it a hundred years ago, predicting we'd be working 15-hour weeks by 2030.