There's few things more detrimental to the productivity of your thought workers than the interruption of flow, yet I think that so few teams recognize its importance. Demarco and Lister describe flow as
"a condition of deep, nearly meditative involvement"
Flow is a requirement in what we do. It's extremely productive time that increases quality, creativity and an overall feeling of accomplishment at the end of the day. For most, this state of immersion takes a bit of time to get into, but takes a split second to get out of.
In my experience, email and IM are disruptions that you can more easily tune out, or just flat out remove by turning them off. But the biggest disruption of flow that I've come across is when someone stops by a co-worker's desk with a "Hey, do you have a second?" Instantly the flow is lost. Your second is given to that person, at the expense of 15 minutes more of your time to get immersed again.
Say you have 5 of those interruptions a day (which is probably pretty conservative for most people), you've got nearly and hour and a half of interruptions. Tack on a few meetings, and you've got a half of a day of productive time on what was scheduled at 8 hours.
If you have taken the time to recognize this issue, often times what the response is, is to start padding scheduling. You start to say, "Jim is only productive 50% of the day". This results in timelines being pushed out or features being cut, and the overall sense that it takes forever to get stuff out the door. Not only that, but Jim doesn't want to be 50% productive. He want's to be 100% productive. He wants to feel like he knows where his day went.
It's easy to measure your flow. You can count the number of uninterrupted work hours per day. If you make it from 9-11 AM without an interruption, you've got 2 hours of uninterrupted flow. I'd challenge you to try measuring that with your team. I'd bet you'd be surprised at the ratio of uninterrupted hours to hours worked.
What can you do to curb interruptions? Designate certain hours in the day when people are "publicly available". Organize your space so that it's conducive to flow and impedes interruption.
If you need to make a case to higher-ups on the impact of flow, you can try measuring a developer who has usually low uninterrupted work hours, and put them up in an office for a week. Measure their uninterrupted work hours then. Say that they usually have 3 uninterrupted work hours for every 8 they work when they aren't in an office, but they've got 6 uninterrupted for every 8 that they work when they are in an office. By rearranging you've made them 50% more productive.
Protect the flow.