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Seasonal affective disorder (SAD) is a mood disorder marked by recurrent episodes of depression that occur during certain seasons of the year, particularly late fall and winter. In rare cases, patients may instead experience these symptoms during late spring and summer. Patients with SAD typically find that their depression lifts as soon as the troublesome season passes.

The exact cause of SAD is not fully understood. However, it appears that a decrease or absence of daylight plays a significant role in most cases. People who live in northern climates with longer nights and shorter days during the winter, or in an environment with frequent overcast skies, are especially at risk for SAD. The condition also affects women more often than men.

The depression associated with SAD tends to be mild to moderate in severity. Patients may experience symptoms that include daytime fatigue and lethargy, excessive sleep, weight gain and a strong desire for sugary or starchy foods. Patients who have summertime depression – also known as “reverse SAD” – may experience anxiety, decreased appetite, insomnia, irritability and weight loss.

Patients who feel depressed for any length of time or who feel that life has gradually become less pleasurable are urged to see a physician. Before diagnosing SAD, the physician will rule out other medical conditions that cause similar symptoms.

Light therapy is the chief treatment for SAD. In this therapy, patients sit before bright, fluorescent bulbs that are designed to concentrate the light on the lower half of the retina, an area of the eye that has photoreceptors involved in antidepressant response. Sessions typically last for 30 minutes or longer and are extremely effective for most patients.