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Seasonal affective disorder (SAD) is a mental health condition that causes recurrent episodes of depression at certain times of the year. People with SAD are most susceptible to depression during late fall and winter, when a decrease or absence of sunlight appears to negatively affect a person’s mood. Patients typically experience normal moods during spring and summer, when daylight is more abundant. 

Many people who live in cold winter climates experience temporary feelings of sadness, lethargy or similar emotions during this season. This is commonly known as “cabin fever” or the “winter blahs.” However, SAD is a condition that causes the patient to experience significant depression that interferes with their personal relationships, work performance and quality of life. Estimates have found that between 4 and 6 percent of people in the general population suffer from SAD, according to the American Academy of Family Physicians.

Some people with SAD may experience symptoms in the summer, with normal moods in the winter. This is called “summer SAD” or “reverse SAD.” Another rare form of SAD involves depression that occurs during the summer and winter, but lifts in the spring and fall.