Does a short nap improve learning efficiency?
Naps may help if you are so sleepy that you can't focus in learning/training anymore. More importantly however, even a short sleep gives start for the memory consolidation process. So all the things you learned before the nap, get better shielded against the new conflicting or overlapping information in future. Especially in procedural learning (e.g. fingers on guitar chords), which is slow, gets important boost by a training -> consolidation -> training -> consolidation loop. Deep sleep (slow wave sleep at night) is more important, but already nap with light sleep phases (state II with k-complexes) will help.

You can search research papers for "memory consolidation" and "sleep".