BBC Online: 'Infomania' worse than marijuana
Workers distracted by email and phone calls suffer a fall in IQ more than twice that found in marijuana smokers, new research has claimed.
I assume they're referring to a temporary "loss" while you're in a state of being distracted and dealing with the interruption... Otherwise I'd probably have a negative IQ by now ;)
But it does underscore my comment the other day about setting a schedule and sticking to it, and include one or more slots for dealing with e-mail in your schedule.