Ferris Jabr, writing for Scientific American:
In one of the few controlled eco-sychology experiments, Berman asked 38 University of Michigan students to study lists of random numbers and recite them from memory in reverse order before completing another attention-draining task in which they memorized the locations of certain words arranged in a grid. Half the students subsequently strolled along a predefined path in an arboretum for about an hour whereas the other half walked the same distance through highly trafficked streets of downtown Ann Arbor for the same period of time. Back at the lab the students memorized and recited digits once again. On average, volunteers that had ambled among trees recalled 1.5 more digits than the first time they took the test; those who had walked through the city improved by only 0.5 digits—a small but statistically significant difference between the two groups.
It's okay to take a break during work and go for a walk through your local park. You'll be smarter afterwards.