Seasonal Affective Disorder Geography

Because SAD is traditionally associated with the lack of sunlight during the long winter months, researchers are continually investigating the link between SAD and geographical location. According to Northern County Psychiatric Associates (2002), the incidence of SAD in a population generally increases as you move further from the equator. Those living in Nordic countries — which experience some of the world’s longest winters and shortest daylight hours — are more likely to be affected by SAD than those in sunnier climates.

Most people who suffer from seasonal affective disorder experience symptoms of SAD every year from fall into winter. Experts believe that the reduction in sunlight triggers neurological and hormonal changes in the body that cause the symptoms of SAD, from anxiety to appetite changes.

In rare cases, SAD occurs as a result of high temperatures and humidity during the summertime. Little research has been done on this so-called “reverse SAD” and its geographic link.

The Biology of Seasonal Depression

While the exact cause of seasonal affective disorder is unknown, it’s been linked to a chemical imbalance in the brain triggered by shorter daylight hours and the relative lack of sunshine in winter. Specifically, reduced sunlight can cause a drop in the neurotransmitter serotonin, which affects mood. Drops in serotonin levels have been connected with depression symptoms.

According to the National Mental Health Association (2004), the change of seasons also disrupts the balance of the natural hormone melatonin, which plays a role in sleep patterns, as well as mood. Increased melatonin levels have been linked to the symptoms of depression.

Geography and SAD: Ongoing Studies

While it may seem obvious that northern populations have a higher incidence of SAD than their southern counterparts, the connection isn’t as clear as it may seem. A study published in the American Journal of Psychology (2009) reported that the Icelandic population had a disproportionately low incidence of SAD, despite long winters and minimal sunlight exposure for months at a time. A related study found that Canadians of Icelandic descent seemed similarly immune to SAD despite the climate. These studies have led some researchers to believe that certain populations may be less susceptible to SAD symptoms due to diet, lifestyle or genetic factors.

Another study, published by the National Institutes of Health (2009), surveyed Italian and Norwegian students on mood and behavior over the course of a year. This study found that the prevalence of SAD between the two countries was quite similar, and didn’t conclusively link SAD to geographic latitude.

Resources

American Academy of Physicians. (2010). Retrieved July 14, 2010, from http://familydoctor.org/online/famdocen/home/common/mentalhealth/depression/267.printerview.html.

American Psychiatric Association. (2010). Seasonal affective disorder. Retrieved July 10, 2010 from http://healthyminds.org/Main-Topic/Seasonal-Affective-Disorder.aspx.

Axelsson, J., Karlsson, M. M., Magnusson, A.