This is anecdotal, so not data, but I'm not 100% sure i agree. But it may be an age thing.

In my youth (for some definition of youth) I could work flat out for 10 hours+. I'd get in the groove and just fly.

I'm a little (or lot) older now - and I can do maybe 6 hours at a time now. I now fill the rest of the time with non-code work, like docs etc. Or I just knock off early.

I have 30 hours workweek and work 5x6, for the last 7 years. I also negotiated that I can leave at noon, and work the remaining hours, from where ever I am, when there is need (e.g. when I need to answer crucial things at evenings, weekends etc., which happens maybe once in a month). It is very good, I can spend more time with my son. I can go running when there is still sunlight outside. The problem is I could not go back to an 8 hour day.

I don't abuse this deal: When I really want to get things finished, or when there's something critical, of course it happens that I am still at work Friday 4PM. It works the other way, too: When there's something critical at home (and no planned interaction at work), I may not show up at all, and it is Ok.

Member of my team moved recently to 4x9.375 instead of 5x7.5. My company wouldn't allow an hours reduction at the same or similar salary, so he took the offered option above. He takes Wednesday off and seems to enjoy it. I'm considering the same now, too.