When I study, I like to do it in blocks of a few hours, with little rests in between. I have heard from multiple (possibly unreliable) sources, however, that a human's concentration will drastically drop after 45 minutes.
I find it doubtful that this limit should be the same for all humans. If there is a limit, I find it doubtful, to be so low, since there are people with jobs that require much more concentration than that (also, exams usually last longer than that, requiring people to concentrate for a few hours straight). I also doubt that this limit can't be improved by exercising your brain regularly or possibly by some sort of meditation. My question is therefore: Is there any research behind this claim?
A google search quickly reveals the ubiquity of this claim, so I suspect it falls in the category of "common psychological wisdom":
On Yahoo! Answers the chosen answer advocates partitioning your studying time into intervals of 45 minutes.
"Don't try to work for 10 hours straight. Schedule 45 minute blocks with 15 minute breaks. Decide what you are going to accomplish during those 45 minutes, and record what you have done."
But this claim from the same source (user JAMES) is even more interesting:
"Hey friend,,, i wanna tell you a fact,,, that it is scientifically proved that human brain can concentrate on one thing only for 40 - 45 minutes (not more than that,,,, it is the maximum limit)...."
On wikiHow's How to Finish Your Homework we find the same claim (see Tips):
"Your homework may take several hours to complete. You must take a 10-15 minute break every 45 minutes. Your brain can only concentrate for 45 minutes."
On WebAnswers, again (user Leandraluv):
"Right, get back to your checklist and start studying. Take a break every 45 minutes to one hour or so to get some water and take a little walk. It makes no sense if you keep going for three hours straight .. after some time you won't absorb any information."
"Continuous studying is not a recommended habit as brain needs rest in every 40-45 minutes thus make sure you give yourself breaks while studying. A 15 min break for a 45 minute continuous learning is enough."
I realize none of these sources are very reliable, but I'd say this is strong evidence of the claim being completely ingrained into the collective consciousness of our society.