There's a great technique for increasing your attention span that was supposedly developed by Albert Einstein. It's called the Einstein Distraction Index, though I wouldn't really consider that proof! The steps are fairly simple:
-Take out your textbook and place beside it a notebook with a plain page.
-Write at the top of the notebook "I will not be distracted by that."
-Set a timer the 3 minutes and begin reading your material.
-Any time a distracting thought pops into your head, read the quote at the top of your notebook and put the tally beside it.
-When you can get through the entire 3 minutes without a single tally, move the timer to 6-minute intervals.
I'm afraid I can't link you do any research that proves the validity of this technique, I can only talk from my personal experience. The Einstein Distraction Index definitely enhanced my ability to maintain my attention over long periods.
The other benefit is, once you have completed a certain interval without any distracting thoughts, you won't need to perform this exercise for another 6 weeks in order to keep the benefits.