Have you had this experience?
“I don't understand it. Yesterday I learned this passage and I could play it perfectly--even up to tempo. But now, at my lesson, it is as though I have never seen it before!”
This is a natural learning phenomenon, similar to the shallow kind of learning that takes place when one “crams” for an exam. Anything “learned” in this way, using short-term memory only, is soon forgotten.
Long-term learning requires repeated and frequent exposure to a concept or activity over a period of time. Alternating disciplined practice time with rest time builds long-term memory. The technical matters required to play a piece then soak into the subconscious, leaving the conscious mind free to focus on making music.
This means that regular daily practice is the most effective, at the time of day when you are most alert. Also, the most effective two-hour practice session is one that includes short breaks. Many organists feel great benefits from a five-minute break every twenty-five minutes.