Multitasking Makes You Stupid and Slow
 

Multitasking messes with the brain in several ways. At the most basic level, the mental balancing acts that it requires — the constant switching and pivoting — energize regions of the brain that specialize in visual processing and physical coordination and simultaneously appear to shortchange some of the higher areas related to memory and learning. We concentrate on the act of concentration at the expense of whatever it is that we're supposed to be concentrating on... studies find that multitasking boosts the level of stress-related hormones such as cortisol and adrenaline and wears down our systems through biochemical friction, prematurely aging us. In the short term, the confusion, fatigue, and chaos merely hamper our ability to focus and analyze, but in the long term, they may cause it to atrophy.

http://www.theatlantic.com/doc/200711/multitasking