Years a go i notice this also. But now im not sire anymore. But i believe its then like whisper, we use less power in out speach because we are tired. I remember also, that often ppl told me im speaking too quiet and powerless. Maybe that was me trying to stutter less. In my smokingtimes, weed madr me stutter less, nowadays a smoke maybe 2-3 times a year. And i stutter more, but also more energized.