Earlier quoted context omitted.
I suspect it's a number of reasons. - I work to my own schedule. There's no artificial start time. If I feel like I've got something to contribute at any point I sit down and do so. If I feel inspired or get in the zone I'll do 6 or eight hours, if I don't I'll do none, or just a couple of hours. - Conversely, if I don't feel up to it, uninspired, tired, whatever, I can stop. Taking a break/nap and coming back to som…
I agree, working 4 hours a day will feel more effective than the last 4 hours of an 8 hour day, but the question is whether it's actually twice as effective. And perhaps it's so for very high level tasks, but I'm pretty sure for some things you just need to clock in the hours, like closing 100 tiny bugs because you're launching in a few days.
I'm lucky in that I can work the hours I need to work. Last week I did days that lasted 4, 6, 1.25, 1.75 and 4 hours (the 1.x days I had other stuff on).