n North America, it is estimated that 50 to 60 percent of the population is affected by winter depression. There is a broad spectrum of symptoms that arise, from a change in sleep patterns and weight gain, to overwhelming suicidal thoughts and the unfortunate results. Think of it this way - fully half of the people you know are somewhat depressed in the wintertime!

Wintertime depression is very common in those parts somewhat north (or south) of the equator. People who live near the equator have a very low incidence of winter depression. Why? Simple. They live where there are enough daylight hours to avoid triggering the hormone imbalance that causes depression. Now, please note that I did not say that people near the equator spend more time sunbathing than northerners. It doesn’t matter that they have a better tan. Rather, it is the length of the day that matters.

Again, it is the length of the day, not the weather! This fact about winter depression is what gave rise to the name Seasonal Affective Disorder. People who live in northern climes suffer because of the seasonally shortened days. Just as trees lose their leaves when the days shorten, people lose their happiness. People who live where the days shorten seasonally are victims of the brain’s chemical reaction to lesser hours of daylight.
Depressie Boek