http://www.johndcook.com/blog/2013/02/04/four-hours-of-concentration/
And since this is the Internet, and facts are involved, our gwern turns up there also.
Many thinks that they can concentrate for much more hours, every day. Perhaps they don't know what does it really mean, to concentrate.
Thats a completely unfalsifiable claim
http://www.johndcook.com/blog/2013/02/04/four-hours-of-concentration/
And since this is the Internet, and facts are involved, our gwern turns up there also.