[Michielvv]

Exactly the same experience here. I always felt bad about those periods that I wasn't able to even put in 20 productive hours a week, until I started writing detailed hour log: every bad period was always preceded by a very productive period where everything seems to come together.

Even the last three weeks: I was stuck on a design problem and couldn't really move forward/get productive for two weeks. Then when the solution finally clicked you can have a few days of just coding it all, that feels so much more productive.

Also got similar experience with being able to easily work 16 hour days on tasks that always have a clear and small next step, like CSS/design fixes.

What I've learned is to take advantage of those days that everything just works out and make the extra hours and not feel guilty when they don't. Works much better for me than trying to stick to a fixed number of hours per day.