Sleep is a given. The brain is learning continuously, not just during sleep. Go back to a section of a piece after 15 minutes doing something else and notice it has become easier.
To complete the argument about learning: you play the tough section, correcting a few errors. Then you go away for a bit, and during that period the brain updates its hardware to include those corrections. The mistake is to hammer/saw away repeatedly and interfere with what you have already achieved, wasting time and effort. I think the OP kinda acknowledged this by recommending switching at whim between different tasks within a practice session.