Tasks that require a high cognitive load, such as thinking about a problem, reading a research paper or writing C++ code, are very sensitive to interruptions. I find that when I am interrupted during such activity, say by a phone call, or by a co-worker asking me something, the net cost of the interruption is not just the amount of time I spend attending to the call or person. It's as if I have a short term RAM in my head, and I have to re-load the entire context of what I was doing when I get back to it. And this can take a lot of time. What I like are large chunks of uninterrupted time, not several small chunks, even if they add up to the big chunk. However, shutting oneself off from all interactions with others at one's workplace is not the solution, because one runs the risk of not interacting enough with colleagues. Richard Hamming said about Bell Labs:
I notice that if you have the door to your office closed, you get more work done today and tomorrow, and you are more productive than most. But 10 years later somehow you don't know quite know what problems are worth working on; all the hard work you do is sort of tangential in importance.
I have personally found that conversations over coffee and lunch have led to much more interesting directions in my work than solitary thinking. While Hamming was speaking about a research lab environment, I suspect his statement holds true in other creative domains as well.
The apparently contradictory requirements above can be resolved by planning out both solitary time and interaction time during the work week. Both are necessary. I feel creative work requires a combination of two distinct phases. One that allows focused concentration on a task at hand, and another for free discussions, exchange of ideas, and making oneself available to others as a sounding board. Exploit and Explore. An ideal workspace should provide for both. It is said of the original Bell Labs building, that its designer deliberately created long corridors, so that researchers would be forced to encounter one another. Steven Johnson explains in his book 'Where Good Ideas Come From' that coffee houses played a crucial role, because they were breeding grounds for collective thinking.
During my travels on the Internets, I have come across a number of writings on the impact of interruptions on productivity that I can attest to from my own experience:
Paul Graham writes about meetings:
I find one meeting can sometimes affect a whole day. A meeting commonly blows at least half a day, by breaking up a morning or afternoon. But in addition there's sometimes a cascading effect. If I know the afternoon is going to be broken up, I'm slightly less likely to start something ambitious in the morning. I know this may sound oversensitive, but if you're a maker, think of your own case. Don't your spirits rise at the thought of having an entire day free to work, with no appointments at all? Well, that means your spirits are correspondingly depressed when you don't. And ambitious projects are by definition close to the limits of your capacity. A small decrease in morale is enough to kill them off.
Joel Spolsky writes about context switching between different projects.
Some interruptions are self created. Compulsive checking of email for instance. Or deciding to multitask, or trying to handle too many disconnected projects. I find I can do one major thing per day, if I try to do 2 or more major things, I risk accomplishing none. Having smartphones set to beep when an email arrives doesn't help. I switched my phone to not auto check email, long ago.