Does a short nap improve learning efficiency? When one attempts to learn a new skill (e.g., play a guitar), it is often heard that the first hour of practice is more effective than the second. I have heard some people say that even 10 minutes of practice may be effective when you do so everyday.

On many (non-scientific) websites I have read that taking a little nap of 15-30 minutes during prolonged practice is enough to maintain effective learning. Is this true, or is more sleep required? Or is sleep irrelevant altogether?
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".