One thing I have learned through working in project-based environments is that humans are absolute crap at estimating their own endurance levels. We're also crap at remembering other people are humans too.
Loads of times there's a situation where a higher-up screams, "but it has to get done!", as if the laws of physics will bend to their will if they say it emphatically enough. So people (sometimes including me) wind up pulling all-nighters attempting a last-ditch effort. Usually we get something completed; it's just not completed very well. By Hour Ten we're starting to lose the plot. By Hour Fourteen one team member winds up in charge of Remembering What the Hell Task We're On Right Now.
Then morning comes, and the code gets run, and (surprise!) there are mistakes in it, and we wind up fixing them over the course of a day or two. Typically it takes about a week to get things in good shape. Coincidentally, the number of days to fix everything often matches the number of days of work the team estimated in the first place, before heroic measures were invoked.
What's going on here? And what does it have to do with writing?
Writing a text like a novel is projectised work. There's a kickoff (an idea!), a construction phase (a word count!), and a refinement phase (edits!). Eventually, you might even get to publish (deployment!).
And yes, having deadlines and pushing yourself and sticking to your work are all ways to get there. Deadlines are good.
Having unachievable deadlines is bad.
If you're assuming you'll be as fresh on the sixth straight hour as you were in the first, you're setting yourself up to fail. If you decide on a word count goal that can only be achieved by living on so little sleep you're not awake enough to drive safely, you're setting yourself up to fail.
Now, I'm not against a little writerly boundaries-pushing. I've had some amazing ideas about half an hour after I should have gone to bed. At that point, I'm so tired that the characters just come out and talk to me as if they're sitting in the living room with me. Likely I'm so wiped out I'm half-dreaming them already. That's all fine and well for first inspirations, or having some fun with automatic writing.
But for the long haul, you have to keep your strength up, because if you don't, the writer's block is going to come down like a hunk of granite, if some other aspect of your health doesn't fail first.
Writers have to watch this especially because we are both the project executor and the hands-off manager. Unless we're actually under contract to a publisher, we're the ones calling the shots on ourselves. We have to remember that while our taskmaster self may want the next five chapters edited and done by next Wednesday, our writer-self may only be able to pull off two before burning out. On the other hand, if things aren't moving forward at a certain rate, inertia will overcome momentum.
Right now, I'm trying something that's new(ish) to me — doing a book breakdown before I get too far into the words themselves. It's an interesting approach because it's closer to what I do for my day job on software projects. Next week I want to walk through the series of spreadsheets (yes, spreadsheets) I've been filling out.