Seasonal affective disorder (SAD) is a subtype of major depression characterized by onset at a certain time of year, usually the winter. First defined in 1984, SAD is also called 'seasonal depression,' 'winter depression,' or 'major depression with a seasonal pattern.' Each of these terms refers to a subtype of major depressive disorder.
Individuals suffering from SAD have all the typical symptoms of depression, including low mood, reduced interest, decreased concentration, low energy and fatigue. In addition, however, they also tend to have a specific symptom cluster comprised of
Long-term follow-up studies of SAD have found that over 60 percent of individuals diagnosed with the disorder continue to demonstrate a seasonal disturbance of mood and/or behaviour over time. Approximately 20 percent of people with SAD can have complete remission within several years of first diagnosis. The stability of the diagnosis of SAD seems to be similar to the long-term stability of the diagnosis of major depression itself (i.e., 44 to 76 percent of individuals with major depression maintain the diagnosis over several years of follow-up).
The majority of people with SAD have unipolar depression, but as many as 20 percent may have or go on to develop bipolar depression. Typically the manic or hypomanic episodes occur in the spring and summer, and it is critical that these episodes be distinguished from the improved mood related to the remission of SAD for that season. There are important treatment differences for individuals with bipolar as compared with unipolar illness.
In Canada, the prevalence rate of SAD is between 2 and 3 percent. In the United States, however, the rate is less than 1 percent. European community-based studies have estimated the prevalence of SAD to be between 1.3 and 3 percent of the European population, whereas studies in Asia report rates of 0 to 0.9 percent of their population.
Women are more likely to suffer from SAD than are men, with an
average ratio of approximately 1.8 to 1.
However, some studies place the female to male ratio much higher, at 4 to 1.
The lifetime prevalence of SAD increases with age until people are in their fifties. After the age of 50 to 54, prevalence rates decline dramatically, and the occurrence of SAD in people over the age of 65 is very low.