Seasonal Affective Disorder is a type of depression which occurs during the winter months to over a half a million people. From the month of September through April, in some parts of the world, is when the depression begins and ends. The cold months of December, January, and February tend to be the worse for a sufferer of SAD.
Due to sunlight being less throughout the winter months, SAD is brought on by lack of sunlight or natural light. A person between the ages of 18 and 30 can first experience SAD. If the depression occurs for more than three winters, this can be a sign their body is suffering from lack of sunlight.