I've termed Joel Spolsky as The Easiest Man to Disagree With Who Still Meant Well, but there are a number of places where I agree wholeheartedly with what he has to say. There's one quote in particular about getting into "the zone" while working that I find myself digging back up, again and again. I'm adding a quick-link to freakinname's template just to save from searching for "interruption" on Joel's site again.

Anyhow, here are the important parts of his post:

Here's the trouble. We all know that knowledge workers work best by getting into "flow", also known as being "in the zone", where they are fully concentrated on their work and fully tuned out of their environment. They lose track of time and produce great stuff through absolute concentration. This is when they get all of their productive work done. Writers, programmers, scientists, and even basketball players will tell you about being in the zone.

The trouble is, getting into "the zone" is not easy. When you try to measure it, it looks like it takes an average of 15 minutes to start working at maximum productivity....

With programmers, it's especially hard. Productivity depends on being able to juggle a lot of little details in short term memory all at once. Any kind of interruption can cause these details to come crashing down. When you resume work, you can't remember any of the details (like local variable names you were using, or where you were up to in implementing that search algorithm) and you have to keep looking these things up, which slows you down a lot until you get back up to speed.

Here's the simple algebra. Let's say (as the evidence seems to suggest) that if we interrupt a programmer, even for a minute, we're really blowing away 15 minutes of productivity. For this example, lets put two programmers, Jeff and Mutt, in open cubicles next to each other in a standard Dilbert veal-fattening farm. Mutt can't remember the name of the Unicode version of the strcpy function. He could look it up, which takes 30 seconds, or he could ask Jeff, which takes 15 seconds. Since he's sitting right next to Jeff, he asks Jeff. Jeff gets distracted and loses 15 minutes of productivity (to save Mutt 15 seconds).

With so many means of non-immediate communication, it's a shame so many people bug immediately. Here are some hints, folk.

1.) Email the person you want to ask a question, and work on something where you aren't stymied for a while.
2.) If your question is going to take a long time to answer, or even if you suspect it might, schedule an hour or so with the person to talk about what you need to know.
3.) For heaven's sake, when somebody's typing, don't bug them. Wait until they stop, zone out, pick up or hang up the phone, walk out of their office (through preferrably *after* they're coming back from wherever), whatever.

If I had a dime for every time I'd been bugged when I was typing with headphones on, bouncing my leg up and down (usually a sign I'm hitting on all cylinders), sometimes even when I'm gesturing with my fingers (while "visualizing" or, granted, occasionally flipping off the computer, which for some strange reason doesn't seem to care), I'd at least be able to get a good burger for lunch today.