Working in 90-minute intervals turns out to be a prescription for maximizing productivity. Professor K. Anders Ericsson and his colleagues at Florida State University have studied elite performers, including musicians, athletes, actors and chess players. In each of these fields, Dr. Ericsson found that the best performers typically practice in uninterrupted sessions that last no more than 90 minutes. They begin in the morning, take a break between sessions, and rarely work for more than four and a half hours in any given day.

http://www.nytimes.com/2013/02/10/opinion/sunday/relax-youll-be-more-productive.html?pagewanted=1&_r=0&nl=todaysheadlines&emc=edit_th_20130210