(See also this post.)
Ernest Hemingway is the archetypical modern writer. He famously spoke of his art as "work"; he described himself as a "professional". "How does he write?" asked Robert Harling in 1954 (CWH, page 83) and got this answer:
In the early morning. Much of my life has been lived in the early mornings. You get going early for hunting or fishing and get into the habit. In any case, my eyelids are thin, I'm told, and it's better for them in the morning. I get up around six, six-thirty, and start work—or to try to—by eight. I work until ten-thirty, perhaps even midday. Then the day's my own. I can forget work.
As an aside, Hemingway clearly inspired Mordecai Richler's approach, even down to this idea that one works, or rather, that one tries to work. Does Hemingway also forget his characters after his work is done for the day? asks Harling. Yes:
Put 'em right out of my mind. I must—if they're to come alive again the next day. Every writer has his own way of working. That's mine. I take a drink before dinner. Afterwards I try not to. That can spoil things. Then, through the night, through sleep, the subconscious works with the characters. They're alive again in the morning. You understand? Ready for work.
Maybe it's true that there are other ways to work. I doubt it. Certainly, everyone should try Hemingway's approach for a few months. (16 weeks is a good test period.) Obviously, as a researcher, you can't put your subject matter out of your mind after lunch every day. But you can forget the particular argument that you are working on, its particular claims and concepts, its particular empirical materials, its particular theoretical themes. (Hemingway might have been writing a story about marlin fishing or the patrons of a bodega, for example; that did not prevent him from going on a fishing trip or from going to the café.) Or you can try to forget them, anyway. Then try to work the next day. Let them come alive.
What Hemingway understood is that writing takes energy ("juice", he sometimes called it). And as his policy of not drinking after dinner suggests, he understood also that mental energy is connected to other forms of energy. You need to manage your energies intelligently if you hope to write well and with reasonable ease. Finally, Hemingway understood that good writing is not based wholly (or even mostly) on conscious mental activity. Most of the "work" gets done by the subconscious.
I have noticed this in my children's increasing mastery of sports. My son plays hockey; my daughter figure skates. It is always remarkable to see the improvement that seems to take place between practices, i.e., when they are not on the ice. Clearly they skate—try to skate—one day, and then their subconscious "works with" the moves they have been practicing, also through the night. Two days later, they make the same moves easily that they could barely execute at the last practice.
I think all competence develops in this way. And progress can be hindered by working yourself too hard, by not taking breaks, by not letting your subconscious catch up. In fact, competence can be destroyed by not shifting between the effort to work and the effort to forget the work. I'm not here just talking about your ability as a writer, by the way. I mean that your knowledge of your subject matter will develop in a more robust and healthy way if you write consciously about for a few hours every day, and then stop thinking about it (what you're writing about) and turn your mind to other things (like reading about related but different matters, or going back into the field, or teaching). The whole trick is to engage with very complex materials every day after having made a serious effort, for a few hours in the morning, to find what Ezra Pound called "the simplest possible statement" of your understanding of a small portion of those materials.