Sort of related is this new find about multitasking being counterproductive. In brief, each time you "switch" from one task to another, you sacrifice a certain amount of time while your brain ramps down from the first task and ramps up for the second. The more complex these tasks, the bigger the sacrifice. So it'd be possible to work a ten hour day, but if you're multitasking badly effectively only put in six hours of work.
Lunchbreaks not included!

The trick seems to be "boxing" your tasks - one hour for dealing with email and nothing else, one hour for doing sales calls, etc.

Posted a bit more on it here if you're interested.

Cheers

Piers