it's hard to tell from the abstract, but it seems to imply that it's the dwindling of new stimuli over time that we perceive as shorter days (which matches my intuition on this).

as in, more and more of our days succumb to routines that are entirely forgettable. these largely automated ruts require little conscious input, so we lose more of the day to nighttime cleansing. the interesting stuff occupy smaller and smaller slices, which feel like shorter days when recalled.