I must second this point. I'm fortunate enough to work a job where, every few months, I manage to shirk all of my day-to-day duties and get to spend a few weeks just hacking on some big, long-term, greenfield project of my choosing.

After seeing how productive I am during those windows of pure, blissful coder-escapism, getting back to the emails and little issues on GitHub and helping my coworkers... it absolutely shatters my productivity. Not because I don't like doing the little things (they give a zen-like satisfaction all their own, like a well-used checklist). It's the constant context-switching. When I do need to code, it's so tough to get into flow, knowing that I'm likely to be interrupted again at any moment by someone needing help with a bug, or some server or client that's on fire.

...

I was also tempted to request an "All of the above", as all of the issues (yes, all — I was tempted to check every item on the list) play into this, for me. Anything and everything can, and does, cause me to get out of flow — unless I isolate myself completely, and truly know that I am isolated. Then I can flow.

This is mine too. Sometimes it's switching from various meetings and calls, other times it's various emergencies, and other times, it's from other random events. On the days where I can shut out all distractions, I find myself very productive. Unfortunately, I can't afford those kinds of days too often, but I do try to optimize my schedule and environment for them whenever I can.

im in the middle of trying to design and program an app and maintain/extend the existing web version. my schedule looks something like this:

monday: app programming tuesday: web programming and management wednesday: app design thursday: app programming friday, saturday: anything, as long as its productive. not necessarily related to this project

i find that having a day to just play around is really helpful in keeping things fresh. if it weren't for that i would go bananas. great learning opportunities as well.