It can be sustainable. Just takes a lot of setup, and it’s not usually constant for long periods. But I regularly hit 9, even more and have been for many years now through 5 startups.

Only happens when you have equity, you like the work, and you have a lot of it in front of you with no managers or meetings in the way.

But I’ve designed my work so far to be just that. I’d never go back to working on a team or project that wasn’t structured that way. I’m at my happiest when I’m in this zone, it’s the most creative and rewarding work.