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?