November 24, 2020
Written by: Rebecca Somach
It is November, and in the Northern Hemisphere the days are getting shorter and colder. For many people, that means adding a couple extra layers of clothing to keep warm, and cozying up to a fire with more hot chocolate than usual. However, for some, this time of year is not one for seasonal joy and winter fun, but instead is a harbinger of anxiety, low energy, eating dysregulation and depression. About 6% of people in the United States1 will experience a predictable onset of depressive symptoms in the winter months. These symptoms go beyond merely feeling ‘down’ that the weather is colder, and are signs of a type of clinical depression called Seasonal Affective Disorder, or appropriately – SAD. Although less common, SAD can also occur during the spring or summer months, in which case the condition is called summer-pattern SAD. To be diagnosed with SAD, a person must show signs of depression that are more pronounced in one season compared to other times of the year.
As with the more commonly known Major Depressive Disorder, SAD is a complicated disorder with many contributing factors. When studying SAD, researchers look for distinguishing psychological, neurobiological or genetic factors that differentiate it from Major Depressive Disorder and other depressive disorders. One of the factors that researchers have examined is the amount of light that individuals get each day. People who live farther north on the planet get less light each day than people who live closer to the equator. Researchers reasoned that if the lack of light were a factor in developing SAD, people at northern latitudes would be more likely to have SAD. To find out, researchers sent a survey to people at four different latitude levels in the United States2 . Researchers found that the prevalence of SAD was positively correlated with latitude, meaning people in the north did have higher levels of SAD compared to those living further south. In all animals, light helps regulate the body, and many biological processes have daily rhythms, also called circadian rhythms. Considering how important light is to regulate the body, it is reasonable to think that if people get less light, some of these processes will be dysregulated. Interestingly, there are some groups of people where the effect of latitude on depression symptoms is greatly decreased. For example, when sending out surveys to groups of people in Canada, researchers found that a group of people that had ancestry from Iceland had fewer cases of SAD compared to other groups of people at similar or more southern latitudes3. Thus, light is not the only factor that determines susceptibility to SAD and there are likely genetic components as well.
What genetic factors might make people more susceptible to developing SAD? Likely candidates are genes that help regulate the circadian clock and those that produce neurotransmitters that regulate mood. Dysregulation in these neurotransmitters would mean that neurons change how they communicate with each other, which can lead to depressive disorders. One such neurotransmitter is serotonin, which helps regulate mood. When serotonin is released from a neuron, a specific protein called the 5-HTT receptor is responsible for taking it back up into neurons. In all people, the binding of serotonin to those receptors fluctuates with the seasons. It is higher in winter compared to summer, which means that there is less active serotonin in the brain. Decreased serotonin signaling is associated with mood disorders, and a common way to treat mood disorders is with drugs that block serotonin reuptake to keep serotonin working for longer. One study4 found that the binding of serotonin to 5-HTT receptors was highest in people that showed severe SAD symptoms, potentially explaining the underlying cause of their depression. There are other neurotransmitters that have shown to be dysregulated in SAD, including dopamine and norepinephrine5. Understanding how these neurotransmitters contribute to SAD symptoms is an important step in figuring out how to treat depressive disorders such as SAD.
Researchers soon hope to use our genetics to more specifically treat disorders of all kinds, including diseases like SAD. For now, treatments for SAD are similar to treatments for Major Depressive Disorder. Treatments include talk therapy, cognitive behavioral therapy and antidepressants. Light therapy is the most common treatment that is specific to SAD. Patients sit in front of a very bright light for 30-45 minutes a day during the fall and winter months. This simple treatment has been found to be surprisingly effective in the short term6. As more is uncovered about the neurobiology of SAD, diagnosis and treatments will hopefully become even more precise and effective. With several neurobiological targets and some therapies, the future of treating SAD is pretty bright.
Cover image from Pixabay User pasja1000
1. Targum, S. D. & Rosenthal, N. Seasonal Affective Disorder. Psychiatry Edgmont 5, 31–33 (2008).
2. Rosen, L. et al. Prevalence of seasonal affective disorder at four latitudes. Psychiatry Res. 31, 131–144 (1990).
3. Magnússon, A. & Axelsson, J. The prevalence of seasonal affective disorder is low among descendants of Icelandic emigrants in Canada. Arch. Gen. Psychiatry 50, 947–951 (1993).
4. Tyrer, A. E. et al. Increased Seasonal Variation in Serotonin Transporter Binding in Seasonal Affective Disorder. Neuropsychopharmacology 41, 2447–2454 (2016).
5. Levitan, R. D. The chronobiology and neurobiology of winter seasonal affective disorder. Dialogues Clin. Neurosci. 9, 315–324 (2007).
6. Virk, G., Reeves, G., Rosenthal, N. E., Sher, L. & Postolache, T. T. Short exposure to light treatment improves depression scores in patients with seasonal affective disorder: A brief report. Int. J. Disabil. Hum. Dev. IJDHD 8, 283–286 (2009).