Autumnal Equinox

Never too late for me to discover that last September 24, 209 was autumnal equinox this year. It marks the beginning of the autumn season in the Northern hemisphere. Here in the Philippines, it starts the cold months which will end on March 21st of the following year. I sensed that I started getting some doses of depression the past few weeks. Why am I sad? Is it seasonal in nature? Is it because I have SAD (Seasonal Affective Disorder)?