I recently followed "Learning how to learn" on Coursera, and they mentioned two main mode of operation of the brain: focused and diffuse mode. In the first mode, you are obviously focused, but it does not allow you to use part of your brain not related to the task at hand. On the other hand, the diffuse mode is activated when you are more relaxed, and allows you to create connections with remote parts of your brain.

So they advise when you are stuck to sleep on it, to allow the diffuse mode to kick in.

If you are interested, I would recommend you to follow the course. Most of the advice are not earth shattering, but I found the explanation on how our brains work when learning quite interesting.