I am not the author of the following paragraph, but I relate to it. I had been slicing my time into tinier and tinier sections just to try and fit everything I wanted to do into my day, and it was making me crazy. My brain just doesn't stretch that way. Turning off the email and doing just one task for a few hours had a huge effect. It was as if finally the gears were grabbing hold and turning my work in a forward direction. It was wonderful, and I employ the strategy of focusing on one task and only one task all the time now.
This is not to say that email and other client interaction isn't part of the work, because it's crucial. (And I enjoy it!) But it seems important to me to set aside a certain amount of uninterrupted time for work that requires maximum focus.