Seasonal Affective Disorder is a mood disorder that is classified by depressive symptoms that occur within the same season each year, usually winter.  It is commonly thought that SAD is the result of biochemical imbalances that occur with shorter daylight hours lessening amounts of sunlight. As seasons change, people experience a shift in their biological internal clock or circadian rhythm that can cause them to be out of step with their daily schedule.  A  second, less common type of seasonal affective disorder occurs in the summer months in individuals who live in warmer climates. This type of depression is related to heat and humidity, rather than light.