Symptoms of winter SAD usually begin in October or November and subside in March or April. Depressions are usually mild to moderate, but they can be severe. The most common characteristic of people with SAD is their reaction to changes in environmental light. Patients living at different latitudes note that their winter depressions are longer and more profound the farther north they live. SAD is often misdiagnosed as hypothyroidism, hypoglycemia, infectious mononucleosis, and other viral infections.
Scientists believe that light therapy works by altering the levels of certain brain chemicals.
Other antidepressants may also be of value. Persons with SAD generally begin with 30 to 45 minutes of treatment in the morning. Light in the evening may cause insomnia. After a week or two, the person with SAD can experiment with reducing or increasing the daily duration of treatment. The most common light source is a full spectrum fluorescent light.