I won't dismiss that a lot of things can happen during sleep that will help you do better in the morning, but a lot of the time I think this kind of thing boils down to performance falling off as you get tired, to the point of eventually negating improvements you make. It doesn't help if you've figured out how to do something if you're tired to execute on it.
The Dark Souls series of video games are known to be hard, especially the bosses. Many people, myself included, have experienced being stuck on a boss and after 20 attempts they give up for the night. Next time you start the game up you beat the boss on the first attempt!
It's hard to know if this is because your subconscious mind kept working on the problem, or if new neural pathways were formed to handle the eye-hand coordination, or, as you suggest, you just came back to the problem refreshed.
Being refreshed, and not frustrated and tired, seems like the simplest explanation for this.
