Seasonal affective disorder (SAD) is a subtype of major depression characterized by onset at a certain time of year, usually the winter. First defined in 1984, SAD is also called 'seasonal depression,' 'winter depression,' or 'major depression with a seasonal pattern.' Each of these terms refers to a subtype of major depressive disorder.

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SAD is characterized by the following four central features:

• Recurrent major depressive episodes that start at approximately the same time every year (e.g., September-October) and end around the same time every year (e.g., March-April).
• Full remission of symptoms during the unaffected period of the year (e.g., May-August). To be considered a full remission, symptoms of the seasonal episode must be absent for at least two consecutive months.
• Over the lifetime course of the illness, there are more seasonal depressive episodes than non-seasonal episodes of depression.
• Seasonal depressive episodes occur in at least two consecutive years.

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