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.
This is true though it may confuse some who currently do but don’t realize it’s not sustainable and leads to burnout. I used to have 8-9 productive hours a day until I realized the cost. I also have a family now and putting those hours would spend me completely and I’d have nothing left for them. Why in the world would I do that? Even if it was for a lot more money I’d probably do it only for a short period of time with a long break in mind for recouping and tending to what I’d have neglected.
If you’re young and trying to prove yourself and are thirsty to learn and all that sure it works but don’t think this is in any way sustainable.