Seasonal Affective Disorder, generally known as SAD, or winter depression, is the most noticeable expression in humans of photoinducability, the changing of physiology or behavior by day length (or more particularly, photoperiod, the number of hours of light in a day). Sufferers of SAD, which have occasionally included myself, are generally in good mental health much of the year, but if photoperiod gets too short, they find themselves depressed. When photoperiod increases again, the depression recedes. In Alaska, where days can get very short for extended periods, about a tenth of the population suffers from clinically significant SAD this time of year.
This week, in a lecture by Dr. George Bentley, I learned a bit about how animals measure photoperiod, and this changed my thinking about SAD and the biological underpinnings of the winter festival of lights that appears as so many different holidays in so many different cultures.