Seasonal Affective Disorder Causes

The causes of SAD are complex and not yet fully understood. While the primary cause may be geographic location, genetics seem to play a role in this condition.

The causes of seasonal affective disorder (SAD) are complex and not yet fully understood. While the primary cause may be geographic location, genetics do seem to play a role in this condition.

In fact, researchers have found that those with a family history of seasonal affective disorder are far more likely to suffer from this condition as well, as opposed to those without a family history of SAD.

Body Chemical Linked to SAD

Because genetics play a crucial role in determining our individual chemical makeup, it is thought to also contribute to the development of SAD.

Two possible chemicals in the body that may play a part in SAD are:


The amount of melatonin in an individual’s body may be one factor that contributes to SAD.

Melatonin, a hormone that tells the body when to sleep, is part of a delicate balance of chemicals that create the body’s natural circadian rhythms. A circadian rhythm acts as in internal clock, telling the body when to sleep and when to wake. Disrupting this clock may be one of the causes of SAD.

During the winter months that are characterized by fewer daylight hours, the body often produces increased amounts of melatonin. This alteration of the body’s circadian rhythm can cause feelings of sluggishness and has even been linked to depression.

People who produce too much melatonin or have a sharper increase of melatonin production in the winter may be more likely to experience SAD.


Another natural chemical that may play a role in the development of SAD is serotonin.

Levels of serotonin, a mood enhancing neurotransmitter, vary from individual to individual. Genetics plays a role in determining these levels.

During times of reduced sunlight, the body naturally produces less serotonin. When levels of serotonin are too low, depression can occur. SAD may occur in people who have lower levels of serotonin or in those who experience a more drastic drop in the production of this chemical during the winter months.

Two specific genes, 5-HTTLPR and 5-HT2A, are both linked with serotonin production. Research has been done to determine if these genes are risk factors for SAD. While these genes do seem to be involved in the condition, they have not yet been proven to be the root cause.

Population Studies on SAD

To determine if SAD is genetically caused, researchers have conducted various studies on different populations around the globe.

While these studies conclusively proved that geographic latitude was an important factor, they also pointed to the fact that geography is not the only risk factor for seasonal affective disorder.

In fact, these studies indicated that the North America population has much higher rates of SAD than its European counterpart.

North America actually has twice as many sufferers of SAD. One possible reason for this is the higher level of racial and ethnic diversity found in North America.

Europe’s more genetically homogeneous population may be naturally more genetically resistant to SAD.

Evidence in Our Eyes: Eye Color and SAD

Strong evidence for a relationship between genetics and SAD was found in a 2002 study conducted at Columbia University in New York.

This study explored the link between eye color and SAD. The color of our eyes is determined purely by our genetics.

The Columbia University study found that people with blue eyes suffer less severe symptoms of SAD. This may be due to the fact that lighter colored eyes take in more sunlight than darker colored eyes.

Other Genetic Links

Other studies have shown a relationship between SAD and:

  • ethnic/racial background
  • family history of SAD and other mental conditions
  • twins

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