Your body calls quits, your brain calls quits, or your bladder stages a coup d-etat. The bladder wins all wars eventually, so succumb to your coffee fuelled master. The rest of them however, ask us to quit in less cutthroat a manner.
You can power through, knowing it will cost you the next day or you can quit now and get up incredibly early and finish the work then. The latter is a dangerous one to the uninitiated, as it puts a short and solid cap on your deadline and will be a much harder mark to hit. However, working whilst tired makes for sloppy decision making. And once you're beyond the ability to move a stylus around a page, EVERYTHING is decision making. Is that the right line? Is this the right brush? Does that need a texture? How's my composition. Every brushstroke is a question.
If it ISN'T you're not being economical, and you're wasting time.
I got to that point pretty quickly in tonight's stream, and I vote to stand up and walk away for now. I'm cool with that- my deadline's by the end of the week, and I hit the time I promised myself to stream each week.
I did make sure to get down the important stuff so I know where I'm headed- the lines (although there are some sharkwhale designs I might dive back into) and when I could feel myself starting to drop I just zoomed out and painted over the whole thing a rough jist of what I want the whole thing to look like.
I'm happy to stop here- it's enough to tell future-me where things are meant to be headed.
This is a marathon, not a sprint ;)