I think what you’re describing is definitely a step in the right direction, the brain gives clear signals. If you listen to them you can know when it’s still “not warmed up”, “close to its peak performance” or “almost burnt up”. The latter usually happens before the typical “8 hour day”, even more so before the new-age 12+ hour day. I’d still like to propose a kind of different approach, that of adding breaks while you work.

What I tried doing to extend the work-hours I can get in a single day was to take 20' breaks every 1h40 minutes (5:1 ratio). In the beginning the brain fights it off (“A break? Already? We got a good 10 hours+ left in us!!”) but if you enforce it, over the course of a week or two, the transitions will become much smoother and you’ll feel you can just keep going work-break-work-break for 12 or 14 hours. Even better is that you probably won’t have to, since all of these work segments are at your brains “peak performance” so you work much more efficiently.

Again, this is hard to pull off — but the few times I’ve sustained it for say a month, I was happier, more productive and could easily “turn the switch off” after I called it a day.