Here are some studies showing the benefits of taking breaks

In a 2012 study, people were found to go into a different state of neural processing when they allowed their minds to wander. The authors hypothesize that going into this alternative state provides cognitive benefits when returning to more focused mental processing.

In 2011, a study at the University of Illinois was conducted to determine the effectiveness of prolonged work. To summarize the results, the group that was allowed brief diversions from their routine work performed the best. It's worth checking out for the details.

Even going for a walk has been shown to provide benefits. One study from Stanford University found that when people worked on mental tasks that required imagination, walking aided creative thinking more than sitting.