What is the cause of seasonal affective disorder?

Melatonin, a sleep-related hormone secreted by the pineal gland in the brain, has been linked to Seasonal Affective Disorder (SAD). This hormone, which may cause symptoms of depression, is produced at increased levels in the dark. Therefore, when the days are shorter and darker the production of this hormone increases.

Seasonal affective disorder is probably caused as a result of lack of light. Generally it is seen in the fall and throughout the winter months as the sun fades. It improves in the spring and summer as it stays light longer. This results in changes to the brain chemicals that affect mood. It also may have something to do with low vitamin D levels.