I was watching the Tour de France on TV and at one point there was some discussion of when it was most efficient to work the hardest. The speaker--whose qualifications I forget--said that the riders got the most benefit by working the hardest when the situation was most difficult--riding uphill or into a headwind.
I was thinking about whether this has a parallel for writers. Are we going to get the most benefit by working the hardest when things are not going well? Or are we going to get the most benefit by resting a bit when things are going poorly, and then working hard when things start to flow?
We can't do wind tunnel tests to determine this.
I could argue both ways. A dissertation isn't a bike race, so the metaphor is obviously not going to translate without some difficulties.
A couple of days ago, I wrote about the curtain of misery; and I think that working hardest when the difficulty is greatest is liable to create the greatest pain when working--you'll be putting in the most effort at exactly that time the work is the least promising.
On the other hand, if you're stuck--if you're discouraged, frustrated, then that might be a moment when you can gain the greatest value from the work because, for one, you need to keep working consistently, even through difficulty, to finish a dissertation; and for two, it is often the case that the moment of frustration and difficulty is the moment at which the greatest change and growth comes. My yoga teacher was recently talking about how at the moment of difficulty something beautiful is waiting to burst out into growth.
I listed this post under the headings of consistency, persistence, and momentum. I don't think that there is a clear answer to what tactic is going to work best on writing a dissertation. The dissertation writing journey and progress through it are not so easily measured as speed and energy output are. But I do think that generally we want to be consistent in working, and persistent in the face of difficulties because there is momentum to projects and to writing: by keeping the project alive and moving in your mind, and on paper, you increase the ease with which you can work on it, and you keep your mind and body in the rhythm and habit of working on the project and thinking about the ideas.
When is the best time to work the hardest? I don't have an answer to that. It partly depends on you and what works best for you. But, in the long run, it is the tortoise that you should emulate: stick at it in a regular pace. What is key is to keep the progress and the writing piling up.