You may be referring to ...

  from IN A WRITER'S WORKSHOP

  Twain writes ...

  It was by accident that I found out a book is pretty sure to go tired,
along about the middle, and refuse to go on with its works until its powers
and its interest should have been refreshed by a rest and its depleted stock
of raw materials reinforced by lapse of time. It was when I reached the
middle of Tom Sawyer that I made this invaluable find. ... I could not
understand why I was not able to go on with it. The reason was simple - my
tank had run dry; it was empty; the stock of materials in it was exhausted;
the story could not go on without materials; it could not be wrought out of
nothing.

  ... I took it out one day and read the last chapter that I had written. It
was then tat I made the great discovery that when the tank runs dry you've
only to leave it alone and it will fill up again in time, while you are
asleep - also while you are at work at other things and are quite unaware
that this unconscious and profitable cerebration is going on.

  ppg 198 & 199 MARK TWAIN IN ERUPTION edited by Bernard DeVoto - Capricorn
Books 1968