December 05

When it gets dark, people get SAD (seasonal affective disorder)

Here in San Diego, we get a minimum of 9 hours and 59 minutes of sunlight, even in the dead of winter. Boston and Chicago get closer to 9 hours and Anchorage gets as low as 5.5 hours comparlight day worlded to Miami’s 10.5 hours. Does this dramatic difference in sunlight have any health effects? 1% of people in Florida as opposed to 9% in New England or Alaska suffer from Seasonal Affective Disorder (SAD). Younger adults, and people with a family history of mood disorders have a higher risk of SAD. SAD is also more prevalent in women than men. SAD is characterized by the typical symptoms of depression, like feeling hopeless, not enjoying normally enjoyable activites, having low energy, experiencing changes in appetite and sleep, having difficulty concentrating, and having frequent thoughts of death or suicide [1]. However, to receive a SAD diagnosis, these symptoms must be prevalent specifically in a given season (typically fall or winter). It’s as if people with SAD go into hibernation, as people with SAD oversleep, overeat, and show social withdrawal. 

So, what causes SAD? Why is it that at darker times of the year people deal with depression symptoms? 

One reason may be changes in circadian rhythms. A key part of maintaining normal rhythms is melatonin production. Melatonin is released at nighttime to help you sleep. In mammals that change their behavior with the seasons, like bears who hibernate, melatonin is released for longer during the winter than the summer. One study found that melatonin release in patients with SAD happens for longer in the winter than the summer, whereas healthy controls release melatonin for the same duration regardless of season [2]. This change in melatonin could be directly related to the symptoms of oversleeping seen in SAD. Another study found an association between a specific polymorphism of a gene implicated in circadian rhythms (CLOCK) with higher likelihood of season depression [3]. These data suggest that differences in genes and compounds that regulate our daily rhythms could affect the risk of SAD. 

Another contributor to symptoms could be variations in serotonin. Serotonin is a neurotransmitter that is implicated in other types of depression. Researchers have found that brain serotonin concentration is lower in the fall and winter and higher in the summer in healthy controls. Higher serotonin concentration was specifically related to bright sunlight duration. They also found that turnover of serotonin in the brain was lowest in winter [4]. Another study found that serotonin transporter binding potential, an index of serotonin levels, changes seasonally to a greater degree in people with severe SAD compared to to moderate SAD and healthy controls [5]. However, it’s unclear as to whether serotonin fluctuations are causal of mood disorders like SAD.

Another neuromodulator, B-endorphin, is produced upon low-dose UV exposure in rodents (equivalent to 20-30 minutes midday in summertime as a fair-skinned person). UV-exposed mice had pain-relieving effects- they didn’t try to get away from a hot surface or an object poking them as fast as control animals who received no UV exposure. B-endorphin can enter the brain and act as an endogenous opioid [6]. 

*template_cellUV exposure also increases the concentration of the small molecule urocanic acid in the blood and in the brain.  Urocanic acid is found in cells and cerebrospinal fluid in the brain and is involved in a synthesis pathway of glutamate, a key excitatory neurotransmitter. After UV exposure, urocanic acid levels rose. Subsequently, mice had an easier time learning new movements, and generating long term memories [7,8]. Since many people with depression deal with low energy and difficulty concentrating, urocanic acid production could serve as a protective mechanism during spring and summertime for people with SAD.

Overall, it seems like the total amount of sunlight exposure may be modulating the prevalence of SAD. One large cohort study looked at the relationship between sunlight and cognition in depressed patients. They found that decreased exposure to sunlight increased the probability of cognitive impairment, even when accounting for season [9]. Another study found in hospitalized people with bipolar disorder that morning sunlight (eastern facing rooms) lead to recovering from an episode 3.67 days faster than those people with western facing rooms [10]. A small study (n=10) found that there were statistically significant seasonal patterns in energy in patients with SAD, even when other meteorological features like temperature and humidity were controlled for [11].  This supports the idea that sunlight is the main driver of SAD symptoms [12]. 

How can we treat SAD? 


Currently, SAD is treated with medicine, light therapy, psychotherapy, and vitamin D [1,13]. Selective serotonin reuptake inhibitors (SSRIs) have been approved for use to treat typical major depression


, and also are effective at treating SAD. Based on our understanding of changes in serotonin binding in people with SAD, this treatment makes sense. Light therapy has been around since the 1980’s, consisting of early morning exposure to filtered light that is twenty times brighter than typical indoor ambient light. This is distinct from UV exposure, but could be modulating the melatonin release in SAD patients. Light therapy has also been shown to be effective in non-SAD patients with depression [12]. Psychotherapy, specifically cognitive-behavioral therapy, is used to help people with behavioral activation. Vitamin D levels in the blood have been shown to be low in people with SAD, so vitamin D is sometimes used as a treatment, although the efficacy is contested in the scientific literature. 


What next?

There is still a lot of work to be done to investigate the causal mechanisms of seasonal affective disorder. However, continuing this line of research may lead to better treatment. If you or someone you know needs help, please call 1-800-273-8255 to reach the National Suicide Prevention Hotline in the US. 



[1] Seasonal Affective Disorder. (n.d.). Retrieved from

[2] Wehr, T. A., Duncan, W. C., Sher, L., Aeschbach, D., Schwartz, P. J., Turner, E. H., . . . Rosenthal, N. E. (2001). A Circadian Signal of Change of Season in Patients With Seasonal Affective Disorder. Archives of General Psychiatry, 58(12), 1108. doi:10.1001/archpsyc.58.12.1108

[3] Kim, H., Lee, H., Cho, C., Kang, S., Yoon, H., Park, Y., . . . Kim, L. (2015). Association of CLOCK, ARNTL, and NPAS2 gene polymorphisms and seasonal variations in mood and behavior. Chronobiology International, 32(6), 785-791. doi:10.3109/07420528.2015.1049613

[4] Lambert, G., Reid, C., Kaye, D., Jennings, G., & Esler, M. (2002). Effect of sunlight and season on serotonin turnover in the brain. The Lancet, 360(9348), 1840-1842. doi:10.1016/s0140-6736(02)11737-5

[5] Tyrer, A. E., Levitan, R. D., Houle, S., Wilson, A. A., Nobrega, J. N., & Meyer, J. H. (2016). Increased Seasonal Variation in Serotonin Transporter Binding in Seasonal Affective Disorder. Neuropsychopharmacology, 41(10), 2447-2454. doi:10.1038/npp.2016.54

[6] Fell, G., Robinson, K., Mao, J., Woolf, C., & Fisher, D. (2014). Skin β-Endorphin Mediates Addiction to UV Light. Cell, 157(7), 1527-1534. doi:10.1016/j.cell.2014.04.032

[7] Zhu, H., Wang, N., Yao, L., Chen, Q., Zhang, R., Qian, J., . . . Xiong, W. (2018). Moderate UV Exposure Enhances Learning and Memory by Promoting a Novel Glutamate Biosynthetic Pathway in the Brain. Cell, 173(7). doi:10.1016/j.cell.2018.04.014

[8] Chantranupong, L., & Sabatini, B. L. (2018). Sunlight Brightens Learning and Memory. Cell, 173(7), 1570-1572. doi:10.1016/j.cell.2018.05.044

[9] Kent, S. T., Mcclure, L. A., Crosson, W. L., Arnett, D. K., Wadley, V. G., & Sathiakumar, N. (2009). Effect of sunlight exposure on cognitive function among depressed and non-depressed participants: A REGARDS cross-sectional study. Environmental Health, 8(1). doi:10.1186/1476-069x-8-34

[10] Benedetti, F., Colombo, C., Barbini, B., Campori, E., & Smeraldi, E. (2001). Morning sunlight reduces length of hospitalization in bipolar depression. Journal of Affective Disorders, 62(3), 221-223. doi:10.1016/s0165-0327(00)00149-x

[11] Albert, P. S., Rosen, L. N., Alexander, J. R., & Rosenthal, N. E. (1991). Effect of daily variation in weather and sleep on seasonal affective disorder. Psychiatry Research, 36(1), 51-63. doi:10.1016/0165-1781(91)90117-8

[12] Keller, M. C., Fredrickson, B. L., Ybarra, O., Cote, S., Johnson, K., Mikels, J., . . . Wager, T. (2005). A Warm Heart and a Clear Head: The Contingent Effects of Weather on Mood and Cognition. Psychological Science, 16(9), 724-731. doi:10.1111/j.1467-9280.2005.01602.x

[13] Melrose, S. (2015). Seasonal Affective Disorder: An Overview of Assessment and Treatment Approaches. Depression Research and Treatment, 2015, 1-6. doi:10.1155/2015/178564


[1] R/dataisbeautiful – [OC] Hours of daylight as a function of day of the year and latitude.

[2] Chantranupong, L., & Sabatini, B. L. (2018). Sunlight Brightens Learning and Memory. Cell, 173(7), 1570-1572. doi:10.1016/j.cell.2018.05.044

[3] Seasonal affective disorder. (2019, December 05). Retrieved from