Seasonal affective disorder is not yet accepted as a distinct psychological disorder. Rather, other mood disorders (major depression, bipolar depression, dysthymia) can be qualified with a "seasonal pattern specifier." What this means, is that the primary mood disorder appears to vary according to the seasons of the year. Whether this represents a distinct psychological disorder, or simply that seasonal variations in weather and sunlight can trigger a depressive or manic episode, remains to be clarified. But, for the many people who become depressed during the winter months, seasonal affective disorder provides a rational explanation, and appropriate treatment, for a chronic and debilitating psychological problem.

The primary feature of seasonal affective disorder is a pattern of depressive or manic episodes that occurs with the onset of the winter months.  As the days become shorter, and the weather colder, there is an increase in vegetative depressive symptoms. Individuals eat more, crave carbohydrates, sleep more, experience chronic fatigue and gain weight. In pronounced cases, significant social withdrawal occurs as well. Some have described the pattern as a hibernation during the winter months.

Women are far more likely to be diagnosed with seasonal affective disorder than men. There are many possible reasons for this.  Women with small children are more likely to be isolated during the winter months, while those with careers are not. Men are less likely than women to be home caretakers for small children. Statistics are not available regarding differential rates between working and non-working women. Also, women have higher rates of major depression as well. It is not clear whether the higher rates of seasonal affective disorder are simply part of that gender difference, rather than being evidence of another disorder. 

Light therapy or photo therapy has been recommended for seasonal affective disorder. This is based on the assumption that decreased exposure to full spectrum light may be responsible for chemical changes that trigger depression in some people. The treatment requires the person to sit under a full spectrum light for several hours per day, throughout the winter months. As an alternative, individuals are encouraged to take walks outdoors whenever possible (depending on weather conditions). More recently, some studies have suggested that the full spectrum is not necessary, but that it does require exposure to bright light conditions for several hours per day.