The Incubation Effect: How to Break Through a Mental Block

When you take a break from a problem you rest your mind and when you come back it's fresher, so you do better.
The break doesn't just freshen you up, it gives your unconscious time to work towards a solution.

http://www.spring.org.uk/2012/07/the-incubation-effect-how-to-break-through-a-mental-block.php