I found exactly the same thing and it weirded me out. How did I get better without practicing? Often I would hit a wall after practicing for hours and was not able to play a certain passage, but then the next day I magically could.

I've recently found that reading before going to sleep (while not staying up late reading) opens my mind the next day and I wake up knowing whether I have accepted or rejected a premise of a nonfiction book. It's like I read, don't think about it then, but then as I sleep I make up my mind or have more questions about the things that I read.

It's strange, and I wish I could learn more about this phenomenon.

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.

That's an interesting thought!

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.

The music thing is definitely a real phenomenon. It’s well researched and covered in some detail in “Why We Sleep” by Matt Walker (which is mentioned in tfa, and is a fantastic book.)

The fiction thing is interesting.

I do theatre as a hobby and do the same thing with learning lines. I run lines before bed and before lunchtime naps and I absorb the info far easier.