A study of more than 2000 people there found the prevalence of seasonal affective disorder and seasonal changes in anxiety and depression to be unexpectedly low in both sexes. The studys authors suggested that propensity for SAD may differ due to some genetic factor within the Icelandic population. A study of Canadians of wholly Icelandic descent also showed low levels of SAD. It has more recently been suggested that this may be attributed to the large amount of fish traditionally eaten by Icelandic people. In 2007, about 90 kilograms of fish per person was consumed per year in Iceland, as opposed to about 24 kilograms in the US and Canada, rather than to genetic predisposition; a similar anomaly is noted in Japan, where annual fish consumption in recent years averages about 60 kilograms per capita. Fish are high in vitamin D. Fish also contain docosahexaenoic acid (DHA), which help with a variety of neurological dysfunctions. Other countries
In the United States, a diagnosis of seasonal affective disorder was first proposed by Norman E. Rosenthal, M.D. in 1984. Rosenthal wondered why he became sluggish during the winter after moving from sunny South Africa to (cloudy in winter) New York. He started experimenting increasing exposure to artificial light, and found this made a difference. In Alaska it has been established that there is a SAD rate of 8.9%, and an even greater rate of 24.9% for subsyndromal SAD. Around 20% of Irish people are affected by SAD, according to a survey conducted in 2007.
One study has shown that up to 69% of patients find lightbox treatment inconvenient, and as many as 19% stop use because of this.Dawn simulation has also proven to be effective; in some studies, there is an 83% better response when compared to other bright light therapy. When compared in a study to negative air ionization, bright light was shown to be 57% effective vs. dawn simulation 50%. Patients using light therapy can experience improvement during the first week, but increased results are evident when continued throughout several weeks. Certain symptoms like hypersomnia, early insomnia, social withdrawal, and anxiety resolve more rapidly with light therapy than with cognitive behavioral therapy. Most studies have found it effective without use year round, but rather as a seasonal treatment lasting for several weeks, until frequent light exposure is naturally obtained.Light therapy can also consist of exposure to sunlight, either by spending more time outside or using a computer-controlled heliostat to reflect sunlight into the windows of a home or office. Although light therapy is the leading treatment for seasonal affective disorder, prolonged direct sunlight or artificial lights that dont block the ultraviolet range should be avoided, due to the threat of skin cancer.The evidence base for light therapy as a preventive treatment for seasonal affective disorder is limited. The decision to use light therapy to treat people with a history of winter depression before depressive symptoms begin should be based on a persons preference of treatment. Medication
SSRI (selective serotonin reuptake inhibitor) antidepressants have proven effective in treating SAD.