Subconscious memory sorting maybe. As you go through the cycles of sleep your brain sorts short term into longer term memory by discarding irrelevant or extraneous memories (ie you remember where you parked the car but not necessarily the cars parked around it).

So perhaps part of that sorting around a problem, breaking from the extraneous detail to the heart of issue, in a state of relaxation allows for the solution perhaps?

Interesting side note; lack of sleep in the evening and you will forget where your car keys are in the morning as there is no filtering done and your brain is full of extraneous detail.