According to the fifth edition of the American Psychiatric Association’s Diagnostic and Statistical Manual, seasonal affective disorder is a form of recurrent major depression that has a seasonal pattern.

This means that it occurs at a specific time of year (usually in the fall and winter) and goes away other times of year (usually in the spring and summer).

A major depressive disorder is diagnosed when patients experience five or more of the following symptoms over at least two weeks and must include either a depressed mood or loss of interest:

• Depressed mood (sadness, hopelessness, tears) throughout the day

• Decreased interest or pleasure in most activities

• Significant weight loss or weight gain or decrease or increase in appetite

• Insomnia or increased sleep

• Agitation or slowed thinking or behavior

• Fatigue or energy loss

• Feelings of worthlessness or inappropriate guilt

• Indecisiveness or diminished concentration

• Recurrent thoughts of death or suicide

When seasonal depression cannot be explained by a medical disorder and it interferes with everyday life, it often leads to a diagnosis of SAD.

By contrast, the condition known as the “winter blues” is generally less severe, more common, and often goes away when treated.