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Seasonal Affective Disorder: Blame It on Sunlight and Serotonin

SAD The winter blues are not just a frame of mind; they are also a state of mind. Seasonal affective disorder (SAD) is characterized by a drop in energy level, fatigue, difficulty concentrating, irritability, weight gain, and carbohydrate cravings, among other symptoms. It usually begins by late fall or early winter and ends by late spring or early summer. The neurotransmitter serotonin is believed to play a significant role in SAD.

It Is (and Isn’t) All in Your Head

Although scientists have long suspected an important role for serotonin in SAD, concentrations of this neurotransmitter in the cerebrospinal fluid bathing the brain and spinal cord of people with SAD appear to be normal.

In one study, researchers in Australia sought to determine whether concentrations of serotonin and other neurotransmitters measured from blood vessels draining the brain (a more accurate measure of serotonin concentrations in the brain than cerebrospinal fluid) would provide evidence for the relationship between serotonin and SAD.

In this study, researchers enrolled 101 healthy men between the ages of 18-79 years old. None of the men included in the study had a history of major illness (including depression ) or cardiovascular disease, were currently on any medication, or had previous psychiatric therapy. Over the course of a year, the researchers gained access to blood draining from the brain of the volunteers by taking samples from their internal jugular veins, which are found in the neck. They then measured the blood concentrations of serotonin and 2 other neurotransmitters not believed to be directly related to SAD—dopamine and norepinephrine—to use as controls. The neurotransmitter concentrations were then correlated to seasonal factors (measured daily), such as highest and lowest temperatures, total rainfall, and hours of bright sunlight.

As expected, production of serotonin by the brain was lowest in winter and was strongly correlated with the amount of bright sunlight during the day. Other than bright sunlight, no other seasonal factors had an effect on serotonin production. Concentrations of dopamine and norepinephrine were not influenced by these factors.

The major limitation of this study is that it was conducted on healthy volunteers, and it is not known whether environmental factors would affect people with SAD in the same way.

SAD Can Be Treated

Research draws connections between sunlight, serotonin, and SAD. One common treatment for SAD involves light therapy, which is the use of light to stimulate the brain to produce neurotransmitters. You sit a few feet away from a special light for a prescribed amount of time—usually up to 45 minutes—every day. This light is stronger than ordinary office or household lighting. If light therapy works for you, your symptoms will most likely improve significantly.

If light therapy does not work for you or if your depression is severe, your doctor may prescribe a type of antidepressant called selective serotonin reuptake inhibitors (SSRIs). SSRIs increase the level of serotonin activity in the brain.

Cognitive-behavioral therapy may be used to change your patterns of thinking. This will allow you to notice how you react to symptoms. You will then learn how to change your thinking so that you can react differently. This can decrease the symptoms of SAD.

There are several other ways to improve your mood. They seem to be especially useful in cases of mild depression.

  • Exercise regularly—Numerous studies have shown the influence of regular exercise on mood.
  • Talk to your doctor about supplementing your diet with fish oil—Some studies have linked taking omega-3 fatty acids with improved depressive symptoms.
  • Talk to your doctor about St John’s Wort—This herb has been found to help with depression. There needs to be more research, though, on its effectiveness in treating SAD. Always talk to your doctor before taking herbs and supplements. They may interact with other medications. Also, it may not be safe to combine St. John's wort with light therapy because the herb may make you more sensitive to light, possibly causing damage to your skin and eyes.

Other supplements, such as vitamin D, melatonin, and tryptophan appear to have a limited effect in treating some people with SAD.


Mental Health America

National Institute of Mental Health

Canadian Resources

Canadian Psychiatric Association

Canadian Psychological Association


Antidepressant use in children, adolescents, and adults. US Food & Drug Administration website. Available at: Updated April 13, 2016. Accessed November 13, 2017.

Lambert GW, Reid C, Kaye DM, et al. Effect of sunlight and season on serotonin turnover in the brain. The Lancet. 2002;360(9348):1840-1842.

St. John's wort. EBSCO Natural and Alternative Treatments website. Available at: Updated December 15, 2015. Accessed November 13, 2017.

Seasonal affective disorder. EBSCO DynaMed Plus website. Available at: Updated January 1, 2017. Accessed November 13, 2017.

Seasonal affective disorder. EBSCO Natural and Alternative Treatments website. Available at: Updated December 15, 2015. Accessed November 13, 2017.

Vitamin D intake and supplementation. EBSCO DynaMed Plus website. Available at: Updated April 10, 2017. Accessed November 13, 2017.

2/16/2016 DynaMed's Systematic Literature Surveillance Rohan KJ, Mahon JN, et al. Randomized trial of cognitive-behavioral therapy versus light therapy for seasonal affective disorder: acute outcomes. Am J Psychiatry. 2015;172(9):862-869.