I can't speak for other developers, but I've had periods of low productivity that were similar to what you describe, but they were mostly abnormal, for example

I can't speak for other developers, but I've had periods of low productivity that were similar to what you describe, but they were mostly abnormal, for example when I was mentally "done" with a particular job, but hadn't been able to find something else. That said, I think I've got about 6 hours of real productivity available a day on average, expandable when doing urgent wrote work or for an urgent need. I've done some time on the 80 hour crew and it's bad for you.