We humans, children of the Tropics, have an innate revulsion to the waning daylight of autumn, having learned early in our history that the sun is vital to our survival. In addition, ill equipped to function in the dark, we found ourselves at the mercy of predators which had a distinct advantage during the cover of night.
Though we have since learned to heat and light our shelters, have managed to kill off most of our natural predators and can now refrigerate foods that we import from distant lands, a fear of the dark still resides in our collective human soul. Of course, the intensity of that fear varies among individuals; like other human fears, our fear of the dark has both a deep-seated genetic basis (common to us all and ingrained to favor survival) and a superficial, conscious component, instilled by those who influenced our thoughts and behavior during childhood.
Now, as we head into the shorter and colder days of the year, both our natural and our irrational fear of darkness will have some impact on our mood. While diminished to some degree by modern technology, we sense the dangers of the season and, though we now understand the science of the solar calendar, we retain a subconscious fear that the retreating sun will not return.