After working intensely for hours on a grueling task, do you feel inclined to respond to the first thing that clamors for your attention, no matter how unimportant?

Some tasks are long, laborious, and monotonous such that one actually welcomes a modicum of distraction. Also, just as any voluntary muscle, the human brain gets fatigued by intense application especially when the task is boring and interest lies elsewhere. A break eases this fatigue. According to Nathaniel Kleitman, the human brain works best with a 20-minute break after 90 minutes of work. Others like Dr. Ernest Rossi have noted the 90-minute cycles, with peak focus for about 45 minutes. Tony Schwartz of the Energy Project recommends three 90-minute sessions in 8 hours of work. The verdict is out—don't work for more than 90 minutes at a time on any task.

What if you do go beyond the 90-minute recommendation? Two things happen. First, your productivity drops sharply. Second, you are a sitting target for...