The biological clock controls sleep, hormones, and mood. Sunlight reduction in winter causes changes to the biological clock, resulting in maladjustments in daily life and mood. And mood changes tend to trigger seasonal affective disorder.

State University of New York at Albany

Experts have yet to find a specific cause for developing seasonal affective disorder, but they suspect the following factors to be contributors to the development of the disorder:

* Vitamin D deficiency.
* Altered biological clock.
* Increased melatonin.
* Imbalances in brain chemistry.
* Thinking negatively.


Possible Causes of Developing Seasonal Affective Disorder

Seasonal depression: seasonal affective disorder. (2022, April 10). Cleveland clinic. Retrieved February 22, 2024, from https://my.clevelandclinic.org/health/diseases/9293-seasonal-depression

Seasonal Depression (Seasonal Affective Disorder) Reference 

Altered Biological Clock and Seasonal Affective Disorder

Those suffering from seasonal affective disorder often experience negative feelings, anxiety, and stress about winter. These cognitive distortions are not yet known to be a cause or effect of seasonal affective disorder. Therefore, changing one’s attitude about winter might help prevent seasonal affective disorder.

The Effect of Negative Thoughts on Seasonal Affective Disorder

Melatonin is a chemical produced by the brain to regulate sleep. The absence of sunlight can increase melatonin production in the brain, resulting in feeling tired and sluggish in the winter, and negatively affecting mood. 

Melatonin Effect on Seasonal Affective Disorder

People who are at risk of developing seasonal affective disorder may already have less serotonin production in their brains. In winter, a lack of sunlight further reduces serotonin levels in the brain, contributing to depression. 

Brain Chemical Imbalance and Seasonal Affective Disorder

Vitamin D increases serotonin levels, but since vitamin D comes from sunlight, serotonin levels can drop in the winter, leading to vitamin D deficiency. Vitamin D deficiency can adversely affect serotonin levels, resulting in mood disruption. 

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