Seasonal affective disorder (SAD) is a mood disorder in which people's depression has a clear, seasonal pattern, usually coming on in the winter months. It is though to affect at least 5% of the population and between 60 and 90% of sufferers are women. It shares many of the symptoms of non-seasonal depression such as depressed, despairing or irritable mood, loss of interest in life, feelings of worthlessness, excessive or inappropriate guilt, low self-esteem, indecisiveness, diminished ability to think or concentrate and inability to experience pleasure. However, while conventional depression usually produces loss of appetite, weight loss and insomnia, seasonal depression usually leads to craving for carbohydrates, weight gain and sleepiness. Depression can also affect people's thinking leading to deficits in verbal fluency, visual search, psychomotor speed, attention and working memory but it is not known if seasonal depression has a similar effect on people's cognitive powers. A U.S. study of 93 people found that SAD was highly prevalent (28%) compared to 8.6% for conventional depression. More women than men were affected by SAD and both SAD and depression were associated with higher rates of cognition problems in comparison to people with no affective symptoms.
Sullivan, Brianna and Payne, Tabitha W. - Affective disorders and cognitive failures : a comparison of seasonal and nonseasonal depression American Journal of Psychiatry November 2007, 164(11), 1663-1667