There is something magical about the subconscious. I find this works best if you truly understand the problem, but can't quite reach the answer. For me this happens mostly when playing a musical instrument. If you understand what you want to play and how to play it, but can't quite make it all come together, that's a sign for me to go to bed. Next day, there is almost always a noticeable improvement. Brain seems to do it's linking over night.