[diminoten]

I doubt I have anything new to add to the conversation.

The things that people say "don't work" or "can't be sustained" do work for me and can be sustained for me. To wit, things people say "do work" and "can be sustained" just don't work well for me.

For example, I'm able to jump back into "flow" quickly when someone knocks me out with a question or need, and I find it hard to put something down once I start working on it, until I've reached a satisfactory end point. I find taking intentional and small, frequent, breaks during the day is disruptive and hurts my productivity (though it's probably healthy for my back and eyes, I suppose).

It's just plain silly to walk around saying "40 hours" is the magic number, or "8 hours" is the maximum anyone can work productively. Averages may be around those values, but for individuals, any point along the line is possible.