"Multitasking may be ubiquitous in today’s plugged in, multi-device world, but you’ve probably already heard not everyone thinks just because you can do multiple things at once you that you should.
'A study done at the University of London found that constant emailing and text-messaging reduces mental capability by an average of 10 points on an IQ test. It was five points for women, and 15 points for men. This effect is similar to missing a night’s sleep. For men, it’s around three times more than the effect of smoking cannabis. While this fact might make an interesting dinner party topic, it’s really not that amusing that one of the most common "productivity tools" can make one as dumb as a stoner.'"That means when you're switching between answering emails and doing important tasks for your business, when it comes to mental function, you'd be better off if you were stoned. Or, as another quote from the book highlighted by Barker puts it, "when people do two cognitive tasks at once, their cognitive capacity can drop from that of a Harvard M.B.A. to that of an eight-year-old."
See also: Five Ways to Keep Yourself Focused at Work