The biggest cost — assuming you don’t crash — is to your productivity. In part, that’s a simple consequence of splitting your attention, so that you’re partially engaged in multiple activities but rarely fully engaged in any one. In part, it’s because when you switch away from a primary task to do something else, you’re increasing the time it takes to finish that task by an average of 25 per cent.
The best way for an organization to fuel higher productivity and more innovative thinking is to strongly encourage finite periods of absorbed focus, as well as shorter periods of real renewal.
A single principle lies at the heart of all these suggestions. When you’re engaged at work, fully engage, for defined periods of time. When you’re renewing, truly renew. Make waves. Stop living your life in the gray zone.
http://blogs.hbr.org/schwartz/2012/03/the-magic-of-doing-one-thing-a.html