One way of looking at the problem of efficiency is to fit the most work into the least amount of time.
This isn't quite right, though. Let's try this again.
There are 168 hours in the week. What you need to do is find between 6 and 15 very good hours of writing somewhere in these 168 total hours, where three two-hour sessions, (MWF say) and five three-hour sessions, (MTWTF) are the minimum and maximum. Looked at in this way, the problem is finding the best 10 hour out of 168. The problem, then, is having too much time, not too little. It's a problem of finitude. Let's cross off 56 hours for sleeping. That leaves 112. Much better. Now you only have to choose the best 10% of your waking hours.
There is no point in writing during the time of day when you are feeling the worst. If you work well in the morning, take advantage of that. Since there are plenty of hours in the week, the problem is not really time at all, but energy. A morning hour might be four times better than a 4 p.m. hour.