We don't really understand the causes of seasonal affective disorder (SAD). There are a few concepts we do know that are significant.
Deprivation in light which seems to alter our internal clock, in turn changing our circadian rhythm. Circadian rhythm is a 24 hour cycle observed in animals, plants and humans. It impacts sleep/wake cycles, attention, concentration, appetite, sleep,focus and attention and mood.
Below are some theories that have been put forth to explain seasonal affective disorder.
A Photoperiod is the period of time each day that an organism ( human in this case) receives light. Researchers first began to look at this to determine why people have seasonal affective disorder, which makes sense. The question quickly became why do some people suffer from decreased light while others do not?
Part of this theory is that people with seasonal affective disorder differ in how their bodies release melatonin during the winter seasons. One study provided some support for this hypotheses. It was postulated that these people who suffer from SAD retain an old biological ability like some mammals to track changes in the seasons. The later dawns cause their bodies and clocks to run slow, while this doesn't happen to the rest of us. Because this is happening they are out of "sync" in many ways, and this causes their depressive symptoms. (Zauderer & Ganzer 2015)
Another hypothesis associated with photoperiods pertains to retinal sensitivity.
Light therapy one of the effective treatment for SAD is transmitted through the retina. It was proposed that perhaps those with SAD had a deficit in how they absorbed light during the winter months and that this led to a deficiency in light and similar issues with the biological clock. There was evidence found to support this theory ( Lewy 2007).
Low Vitamin D levels are associated with sunlight and with depression, and found in those who have SAD. We know there is a relationship, but it isn't quite understood. It's good to have your level of vitamin D checked by your doctor and to inquire about it if you are suffering from SAD.
Serotonin is a neurotransmitter ( chemical messenger) in our brains that plays a role in depression. In most of us, we have less serotonin in the winter. This hypothesis suggests that individuals with SAD may experience and even greater decrease in serotonin during the winter. There is good evidence to suggest that serotonin plays a central role in SAD (Rohan 2009).
Recently we have learned that people with SAD may have difficulty in serotonin regulation. During the winter months, they overproduce SERT, a protein that transports serotonin from the synaptic cleft to the presynaptic neuron. The higher the SERT levels the lower the serotonin. ( Swift 2011).
Later dawns shift the circadian rhythms in people who have SAD. Studies have been done to give people melatonin in the afternoon and cause " PHASE advances" or in the morning to cause phase " delays" and some of these studies have shown symptom improvement as scientists corrected the shifts in the circadian rhythms of people with SAD (Lewy 2007).
This is a light sensitive protein we believe comes from the retina that is linked to circadian rhythms, hormones, alertness, and the sleep/wake cycle. Individuals who have two copies of this gene mutation are more susceptible to SAD.( Zauderer, C., & Ganzer 2015)
Rohan, Kelly J. Coping with the Seasons : A Cognitive Behavioral Approach to Seasonal Affective Disorder, Workbook, Oxford University Press, 2009. ProQuest Ebook Central, https://ebookcentral.proquest.com/lib/cmlibrary-ebooks/detail.action?docID=415211.
Lewy, Alfred & N Rough, Jennifer & B Songer, Jeannine & Mishra, Neelam & Yuhas, Krista & Emens, Jonathan. (2007). The phase shift hypothesis for the circadian component of winter depression. Dialogues in clinical neuroscience. 9. 291-300.
Zauderer, C., & Ganzer, C. A. (2015). Seasonal affective disorder: An overview. Mental Health Practice (2014+), 18(9), 21. http://dx.doi.org/10.7748/mhp.18.9.21.e973 Retrieved from http://nclive.org/cgi-bin/nclsm?url=http://search.proquest.com/docview/1784809964?accountid=13217
Swift, Batya (2011)Seasonal Affective Disorder: An Overview retrieved from https://www.psychiatryadvisor.com/depressive-disorder/seasonal-affective-disorder-diagnosis-and-treatment/article/649714/