“Multitasking decreases your memory ability. Each task that you’re
engaged in drains part of your mental energy. This drainage is why
multitasking breeds absentmindedness. Your complete mind isn’t present
when you shift from one task to another and back again. You could say
that your mind is absent.”
http://www.dummies.com/how-to/content/dodging-the-hazards-of-multitasking.html