Seasonal Affective Disorder
Seasonal Affective Disorder is not in itself a diagnosable condition. It can be better thought of as a descriptive term. Seasonal Affective Disorder is often abbreviated SAD, which is appropriate in that it describes a person with depression. More precisely, it describes a person with depression that occurs during particular seasons or times of the year.
A Major Depressive Episode can happen at any time. However in some cases there develops a theme or a pattern where a person may experience Major Depressive Episodes primarily if not exclusively during certain times of the year. Then during the rest of the year the person experiences relief from their depression. This seasonally based theme is where the term Seasonal Affective Disorder comes from.
As mentioned Seasonal Affective Disorder is not a proper diagnosis, but rather a descriptive term. If appropriate, a clinician would provide a person with a diagnosis such as "Major Depressive Disorder with seasonal pattern". Here Major Depressive Disorder is the primary diagnosis, and "with seasonal pattern" is a descriptive term that provides more precise information about an individual's unique case. With seasonal pattern is technically known as a Specifier.
People who experience Major Depressive Disorder with seasonal pattern, or Seasonal Affective Disorder, may experience episodes of depression and periods of relief during any time of the year, though they must be consistent from year to year. The most common presentation is for a person to experience Major Depressive Episodes during either the Fall or Winter and for the depression to lift in the Spring.
While it is possible for a person with SAD to have episodes of depression during the months that they would normally be depression free, this is the rare exception to the rule, as the seasonal pattern should be rather well established and consistent.
For those who may suspect that they have Seasonal Affective Disorder, it's important to rule out any obvious external causes for the pattern of depression. For example, if a landscaper couldn't find work in the winter and regularly became depressed at this time each year as a result, then SAD would not apply.
Seasonal Affective Disorder can also be thought of as an intentionally broad descriptive term. The term itself does not (except perhaps when abbreviated) suggest that the episode necessarily be one of Major Depression. In some instances, the onset of Manic or Hypomanic Episodes may also present with a seasonal pattern.
Finally, younger people and those who live closer to the North or South Pole and further from the Equator are at a greater risk of developing the condition.