Here's something else.
1. Don't measure productivity in terms of hours worked. (In the industrial sense, if you permit my crassness, productivity is not hours worked, but how much is produced per worker per hour. It is efficiency. To measure your output by how much you've worked is opposite of how you should be thinking.) Rather, what you should be looking at is getting things done.
2. The basic unit of time you should think about is the day. You want to increase the number of days per week in which you do something significant on a major project. Doing something significant might be starting a chapter, writing a substantial number of words on it (500-1000), having a single lightbulb type idea that represents a significant breakthrough, submitting an article, finishing something, giving it its definitive form.
So if all you do on your major project on any given day is one of the above, then you are making efficient progress. Today, for example, I began a chapter, and it is only 9:30.