Seasonal Affective Disorder - Part 1

SAD aka Winter Depression is a surprisingly common condition
affecting majority of the population often undiagnosed.

The high number of SAD patients in Kerala
which has a tropical climate is quite surprising.
SAD is highly prevalent in areas where there is
extended winter season like in Nordic countries.

It seems SAD mostly affects south Indians from Nov to Feb.
But Winter is mild and Sunlight intensity is not as low as in Temperate zone.

So, why SAD is so prevalent in South India?
I started digging deep and formulating my own hypothesis.

I googled and looked at yearly sun graph for kerala.
As i expected daylight duration is low from Nov to Feb.
So i know daylight duration is more important than intensity.
It's pretty much like photoperiodism in plants.

Many questions came to my mind.
How such a trait evolved in population?
What evolutionary advantage it brings?

SAD is an evolutionary remnant of Hibernation.
Ice age moulded the evolution of humanity in many ways.
In frigid conditions, food is scarce and risk of death is higher
if the animal freely roams around.
So a depressed animal has survival advantage over freely roaming one in winter.
That might be the evolutionary pressure that lead to development of SAD in modern men.

to be continued...