If the eccentric orbit of the earth around the sun is the cause of Seasonal Affective Disorder rather than the varying amount of sunlight per day, then the yearly cycle of SAD in the southern hemisphere should be affected by the calendar month of the year rather than the shortest day of the year. Remember, the shortest day of the year in the southern hemisphere is on June 21st, a date that is fairly near the aphelion. This would explain the great deal of conflicting data regarding the efficacy of Light Therapy, the standard treatment.
However this is a somewhat anti-intuitive treatment since SAD peaks in February and March when the Darkest Day is all the way back in December.
The current Seasonal Affective Disorder theory of mood swings posits that the hormonal balance regulated by the pineal gland causes disruptions in the normal circadian rhythms. Light therapy is thought to address this daylight deficit by supplementing artificial bright light which stimulates the production of melatonin. However numerous studies now contradict this premise.
The model I propose is more straightforward. It predicts a rising energy toward crescendo peaking around January 4th, followed by a rapid shifting of energy until springtime.
The current model insists that the seasonal shift in mood and health is primarily due to the length of daylight and the angle of the sun in the sky.
If this were true then winter-time depression would increase with higher latitudes. It does not. Also the winter depression rates in the Southern Hemisphere would mirror those of the North. They do not. They seem to conform to another (here-to-fore unexplained) seasonal pattern.
A majority of Third World or Developing Nations are located in the southern hemisphere which has hindered studies of populations regarding Seasonal Affective Disorder. However recently some new evidence points to patterns
that do not coincide with depression studies by latitude in the Northern hemispheric latitudes. More Antarctic studies are called for but must take account of the rugged nature of the individuals who choose to live on the southern pole.