John Le Carré, talking in the Paris Review about working on his first book while commuting to work:
In those days English newspapers were much too big to read on the train, so instead of fighting with my colleagues for the Times, I would write in little notebooks. I lived a long way out of London. The line has since been electrified, which is a great loss to literature. In those days it was an hour and a half each way. To give the best of the day to your work is most important. So if I could write for an hour and a half on the train, I was already completely jaded by the time I got to the office to start work. And then there was a resurgence of talent during the lunch hour. In the evening something again came back to me. I was always very careful to give my country second-best.
He also describes a practice that is surprisingly common among writers: stopping in mid-sentence or task at the end of the day.
I always try to go to sleep before I finish working, just a little bit before. Then I know where I’ll go the next morning, but I won’t quite know what I am going to do when I go. And then in the morning it seems to deliver the answer.
This seems to give your mind the chance to sleep on a problem, and it also makes the next morning’s start easier: it saves you the work of having to make a decision about what you’re going to work on in the bleary early dawn. You can just pour your coffee, take up your pen, and finish whatever you were working on the night before.