Why do we get depressed whenever the cold season arrives, and what does the change of seasons have to do with hormones?

As the seasons change, especially fall and winter, many people feel depressed. This is not just a mood swings, but seasonal depression, which is linked to changes in hormone secretion, such as serotonin and melatonin, and we explore the scientific mechanisms and causes.

 

As the summer sun sets and we transition into the cooler months of fall, the number of people who suffer from depression increases. This is a serious problem that cannot be dismissed as a mere mood swings, and it has become increasingly recognized in modern society due to the growing interest in mental health. In particular, many literary works and popular songs set in the fall and winter seasons reflect the image of falling leaves and cold winds, creating a sense of melancholy and highlighting the impact of these seasonal changes on people’s emotions. Naturally, many people perceive fall and winter as a season of solitude.
It is also not uncommon to find people who suffer from severe and persistent depression as the seasons change, which is categorized as “seasonal depression”. It is characterized by persistent feelings of depression, decreased activity, over-sleeping, decreased libido, increased appetite and weight gain, and, notably, 83% of patients are women. In 1991, in the Russian regions of Siberia and Alaska, it was estimated that 16.2% of the population suffered from seasonal depression, and in Australia in 2004, 5.35% of the population was found to suffer from the condition, suggesting that seasonal depression is far more than a minority affliction and has had a significant impact on the mental health of people around the world.
Nevertheless, is seasonal depression simply a product of the bleak visual and tactile images of the cold season? Unfortunately, not much is known about the direct causes and scientific mechanisms of seasonal depression. However, in recent years, scientists have been adding to the evidence that seasonal depression is a disorder caused by the regulation of serotonin and melatonin secretion in the body.
Let’s take a look at the connection between seasonal changes and hormone secretion, and the potential impact of serotonin and melatonin on depression. First, the changing seasons are related to the amount of solar energy the Earth’s surface receives. Seasons can be thought of as a whole set of weather phenomena that change as the Earth revolves around the sun and each point on the planet is exposed to the sun. In addition to the rotation, the Earth’s rotation around its central axis, tilted at about 66.5° relative to the plane of its orbit, further shapes the seasons. Over the course of the Earth’s rotation, which is a year, different regions of the Northern and Southern Hemispheres experience different solar south latitudes and day lengths at different times. The higher the sun’s southern latitude, the more energy that spot receives from sunlight, with summer being the strongest and winter being the weakest.
Melatonin, meanwhile, is a hormone produced by an endocrine gland called the pineal gland in the brain that helps the body flexibly adapt to day and night. It plays a particularly complex role in the body’s regulation of vision, for example, melatonin regulates the degree of sensitivity to light in the melanocytes of the skin and the retina. The way it’s regulated is simple. By increasing or decreasing its production over time, other actions are also increased or decreased. In this case, melatonin”s production is mainly regulated by the circadian cycle, a cycle that occurs naturally inside the body. This is due to the fact that one of its production processes, N-acetylation, is circadian-dependent. N-acetylation is a reaction in which one hydrogen in an organic compound with an amino group (-NH2) is replaced by an acetyl group (CH3CO-). Melatonin is produced at night because serotonin, another hormone produced by the pineal gland, undergoes two processing steps, N-acetylation and O-methylation, and N-acetylation is the reaction that is most active at night, near midnight.
The definition of night varies depending on the season. The body regulates its circadian rhythm by perceiving the amount of light that is available at different times of the day. Therefore, if the body is exposed to more light than before, even at the same time of day, melatonin production is suppressed. The average melatonin secretion starts at 9 p.m., but during the fall and winter, it starts earlier and ends later, increasing the overall amount of secretion. Unlike melatonin, serotonin is light-dependent, meaning that overall serotonin secretion decreases in the fall and winter months when the days get shorter.
So, what is the relationship between serotonin and melatonin secretion during the different seasons? Serotonin is a neurotransmitter, and a decrease in serotonin causes a decrease in neurotransmission in some way. The exact mechanism of this is not known, but through analysis of numerous patients and trial and error in treatment, doctors have found that a decrease in serotonin is associated with depression, decreased socialization, and loss of appetite. Melatonin, on the other hand, is the hormone that induces sleep and lowers body temperature. In the fall and winter, serotonin activity decreases and melatonin activity increases, suggesting that people will generally feel an emotional downward spiral, while sleeping longer and being less physically active. Of course, the direct cause of how serotonin and melatonin affect depression is still unknown, but if seasonal changes are involved, we can speculate that these hormones may have some connection to the condition.
Modern science has identified many causes and treatments for physical pain. However, the scientific causes of mental illness remain largely a mystery, aside from empirical facts. Is science the only clue to solving the puzzle of the human psyche? If so, is our mind controlled by our body and environment? We don’t know if there will ever be a point where we can answer these age-old questions with a single answer. But for now, at least for the condition of seasonal depression, we have a foundation of science and experience that points to one link: hormones. Hopefully, many more connections will be made and many more people will be relieved of the emotional pain that comes with the colder months.