> When I’m not severely depressed I’m able to do two four hours blocks separated by a break in a day, and maybe squeeze out an extra hour or two on a fantastic day.

I do the same thing, it's always kind of puzzled me how people say they work 10-12 hour days. Realistically, it's probably nowhere close to 12 consecutive hours of deep focus. Sometimes it's easier for me to just take breaks and work whenever I have the motivation. Maybe it stretches into odd hours. Sometimes it's less than expected. Sometimes it's a waste of time because I didn't take the time to step back and gain more perspective. Hours never seemed like a good measurement.

I do think part of it is that when people are in really "high intensity" workplaces, there's social pressure to give the appearance of working hard. That could be part of it. I also think SWE (in SV business culture) tends to be more deliverable oriented as opposed to caring about the hours worked