Here's a beautiful essay by Paul Graham called "Maker's Schedule, Manager's Schedule":
In brief -- Managers work in hour-long blocks through the day; great for meeting people and having a friendly chat. Makers, however (writers, artists, programmers, craftsmen) work in half-day blocks at the minimum. Interfacing the two -- e.g., managers calling an hour-long meeting at some random open slot in their schedule -- cause the makers to completely lose the in-depth concentration on a task they require. Call this "thrashing" or "interrupts" or "exceptions", if you like. This blows away a half or a full day of productive work when it happens.
Great observation, and it rings extremely true in my own experience. One of the reasons I'm so happy to be outside the corporate environment these days.