What causes us to feel depressed in winter? We explore the effects of seasonal changes on our physical and mental health, as well as the scientific reasons behind them.
As the hot summer sun sets and we gradually transition into the cooler days of autumn, the number of people reporting symptoms of depression increases more than usual. This is a serious issue that cannot be dismissed as a mere mood swing, and it is receiving greater attention as interest in mental health grows in modern society. In particular, numerous literary works and popular songs set against the backdrop of autumn and winter evoke a melancholic atmosphere by reflecting imagery of falling leaves and chilly winds, emphasizing the impact these seasonal changes have on people’s emotions. Naturally, many come to perceive autumn and winter as seasons of solitude.
It is not uncommon to find people who experience severe, persistent depression in response to seasonal changes; this condition is classified as “seasonal affective disorder.” It is primarily accompanied by symptoms such as persistent feelings of depression, reduced activity, hypersomnia, decreased libido, increased appetite, and weight gain; notably, approximately 83% of patients are women. Meanwhile, in 1991, it was estimated that about 16.2% of the population in Siberia and Alaska suffered from seasonal depression, and in 2004, it was revealed that approximately 5.35% of the total population in Australia suffered from this condition. This demonstrates that seasonal affective disorder is not merely a type of emotional state experienced by a small number of people, but rather a condition that has had a significant impact on the mental health of people worldwide.
Nevertheless, is seasonal affective disorder merely a product of the bleak visual and sensory imagery associated with the cold season? Unfortunately, little is known about the direct causes and scientific mechanisms of seasonal affective disorder. However, scientists have recently been adding evidence to support the theory that seasonal affective disorder is a condition caused by disruptions in the body’s regulation of serotonin and melatonin secretion.
First, let’s examine the correlation between seasonal changes and hormone secretion, and explore the potential impact of serotonin and melatonin on depression. Seasonal changes are directly related to variations in the amount of solar energy reaching the Earth’s surface. Seasons can be defined as the collective set of general meteorological phenomena that change depending on the degree of exposure to the sun at various points on Earth as the planet orbits the sun. In addition to this orbital motion, the Earth’s rotation—which occurs along its central axis while tilted at an angle of approximately 66.5° relative to its orbital plane—further defines seasonal changes. Based on the Earth’s orbital cycle of one year, regions in the Northern and Southern Hemispheres experience varying solar altitudes at noon and differing day lengths depending on the season. The higher the sun’s altitude at its zenith, the more solar energy a given location receives; summer is the season when this effect is most pronounced, while winter is when it is weakest.
Meanwhile, melatonin is a hormone produced by the pineal gland, an endocrine gland located in the diencephalon, which helps the body adapt flexibly to day and night. It plays a particularly complex role in the body’s regulation of circadian rhythms; notably, melatonin regulates the sensitivity to light in the skin’s melanocytes and the retina. The mechanism of regulation is simple: by increasing or decreasing its production over time, other physiological functions are also increased or decreased. Here, melatonin production is primarily regulated by the circadian rhythm, a natural internal cycle of the body. This stems from the fact that N-acetylation—one of the steps in melatonin synthesis—is circadian rhythm-dependent. N-acetylation is a reaction in which a hydrogen atom in an organic compound containing an amino group (-NH2) is replaced by an acetyl group (CH3CO-). Melatonin is produced from serotonin, another hormone generated in the pineal gland, through two processing steps: N-acetylation and O-methylation. Since N-acetylation is a reaction that becomes active near midnight, melatonin is also produced at night.
Here, the definition of “night” varies depending on the season. Our bodies perceive changes in light levels over time to regulate our circadian rhythm. Therefore, even at the same time of day, if our bodies are exposed to more light than before, melatonin production is suppressed. While the average starting point for melatonin secretion (DLMO) is said to be 9:00 PM, in autumn and winter, this starting point occurs earlier and the ending point is delayed, resulting in an increase in total secretion. Unlike melatonin, serotonin production is stimulated by light. Therefore, during the fall and winter when days are shorter, the total amount of serotonin secreted decreases.
So, what is the relationship between the seasonally varying secretion levels of serotonin and melatonin? Serotonin is a type of neurotransmitter, and a decrease in serotonin causes some form of impairment in neural transmission. Although the exact mechanism behind this has not been fully elucidated, doctors have determined through analysis of numerous patients and trial-and-error in treatment that a decrease in serotonin is associated with depression, reduced sociability, and loss of appetite. On the other hand, melatonin is a hormone that induces sleep and lowers body temperature. In the fall and winter, serotonin activity decreases while melatonin activity increases. This suggests that people generally experience a dip in mood, while longer sleep durations lead to reduced physical activity. Of course, the direct mechanism by which serotonin and melatonin influence depression has not yet been identified. However, if seasonal changes apply here, we can speculate that these hormones have some sort of association with this condition.
With the advancement of modern science, the causes and treatments for numerous physical ailments have been identified. However, the scientific causes of mental disorders remain largely a mystery, aside from empirical facts. Is science the sole clue to solving the puzzle of the human mind? If so, are our minds governed by our bodies and our environment? It is unclear whether a time will ever come when we can provide a single answer to these age-old questions. However, at this point in time, at least regarding seasonal affective disorder, we have discovered, through the foundations of science and the basis of experience, that hormones serve as a connecting link. It is my hope that as more such links are identified, many people will be able to free themselves from the emotional distress that comes with the cold season.
