We get stressed out now by having somebody yell at us in the office or by making a mistake or by losing a bunch of money. These aren't problems that our hunter-gatherer ancestors had. They'd get stressed if a lion came to them or a boulder was rolling towards their living quarters. That kind of stress provoked the fight or flight response.
If you're making a bunch of little decisions - like, do I read this email now or later? Do I file it? Do I forward it? Do I have to get more information? Do I put it in the spam folder? - that's a handful of decisions right there, and you haven't done anything meaningful. It puts us into a brain state of decision fatigue.
We need to blinker ourselves, to better monitor our attentional focus. Enforced periods of no email or Internet to allow us to sustain concentration have been shown to be tremendously helpful. And breaks - even a 15-minute break every two or three hours - make us more productive in the long run.
The conscious mind can only pay attention to about four things at once. If you've got these nagging voices in your head telling you to remember to pick up the laundry and call so-and-so, they're competing in your brain for neural resources with the stuff you're actually trying to do, like getting your work done.