In its most common form, SAD is a serious depression that recurs each year at the same time, starting in the fall or winter, and ending in the spring. Some of the symptoms typical of depression are sadness, anxiety, irritability, inability to concentrate, withdrawal into solitude and loss of interest in life. Other symptoms are unusual sleep patterns, increased appetite and weight gain. Lethargy (feeling extremely tired) is such an important feature of the disorder that the term "Seasonal...