Researchers are studying the biology of seasonal depression in an attempt to understand the underlying causes of seasonal affective disorder (SAD). Although what causes the symptoms of SAD in any given person is not clear, researchers have linked SAD to a biochemical imbalance in the brain triggered by shorter daylight hours and a lack of sunlight in winter months. Researchers have also shown that bright light affects brain chemistry, although they don’t know exactly how.

The three major areas of research on the biology of seasonal depression focus on circadian rhythms, neurotransmitters and genetics.

Circadian Rhythms and Seasonal Affective Disorder

The circadian rhythm Ñ also called the biological clock Ñ influences a person when to sleep or be awake. Reduced sunlight in the winter may disrupt this rhythm, part of which is the production of the hormone melatonin. The body produces melatonin when it’s dark, and research has linked increased melatonin levels to depression.

The two main theories about circadian rhythms in seasonal affective disorder are:

  • Phase-shift hypothesis: SAD results when internal circadian rhythms are out of sync with the external clock or other external rhythms, such as the sleep-wake cycle.
  • Photoperiod hypothesis: Shorter days in the winter, and thus less light, induce the symptoms of SAD.

Research into both theories provides conflicting results. However, substantial evidence supports that some, but not all, people with SAD have phase-delayed circadian rhythms that can be corrected with the properly timed use of light therapy or melatonin supplements.

Neurotransmitters and Seasonal Depression

Neurotransmitters are chemical messengers used to carry information from one nerve cell (neuron) to the next. The body produces many different neurotransmitters for various purposes. Although research into the biology of seasonal depression is looking at various neurotransmitters, two of the most frequently studied are serotonin and dopamine.

Much research supports the role of decreased serotonin (also called 5-HT) in SAD. Serotonin affects mood and helps create a feeling of calm and well-being. Because vitamin D helps to maintain serotonin levels, less sunlight results in reduced serotonin levels, which in some people may lead to symptoms of SAD.

Some research has found decreased levels of dopamine in people with SAD. The many roles of dopamine include regulating behavior and cognition (the process of being aware, knowing, thinking, learning and judging). Decreased dopamine may also lead to decreased serotonin.

SAD Genetics

Although researchers are looking for genes that might be involved in seasonal affective disorder, results aren’t conclusive.

However, according to the Mount Sinai School of Medicine, family history studies have found that 25 to 67 percent of people with SAD had a family history of mood disorders and 13 to 17 percent had first-degree relatives with SAD. These rates are significantly higher than expected in the general population.

Another consideration in the biology of seasonal depression is that SAD may be the result of a combination of the effects of circadian rhythms, neurotransmitters and genetics on a specific person’s body.


Sohn, C., et al. (2005). Update on the biology of seasonal affective disorder. Retrieved May 10, 2010, from the UBC Hospital Mood Disorders Centre website: of SAD, CNS Spectrums.pdf.

Bernson, M. (2002). Seasonal affective disorder: shedding light on seasons and the brain. Retrieved May 10, 2010, from the University of Hawaii website: Affective Disorder Shedding Light on Seasons and the Brain.pdf.

 Posted on : June 14, 2014