The bodily changes associated with the movements and transformations of heaven and earth qi in Korean medicine correspond to what modern medicine describes as biological rhythms. In particular, the suprachiasmatic nucleus, located in the hypothalamus and often called the body's master clock, responds sensitively to light and darkness. It regulates our overall biological clock, including sleep, hormone secretion, body temperature, and appetite.
When daylight hours shorten in winter, melatonin secretion increases and serotonin synthesis tends to decline. Melatonin is a hormone that induces sleep, but excessive or imbalanced secretion can lead to daytime sleepiness and lethargy. Furthermore, when levels of serotonin drop, mood lowers and feelings of sadness or depression can develop. During these periods, the brain craves carbohydrates or sweet foods to help replenish serotonin, as carbohydrates facilitate the absorption of tryptophan, which supports serotonin synthesis in the brain. This mechanism accounts for the winter binge-eating tendencies frequently seen in seasonal affective disorder.
Leptin and ghrelin, the primary hormones regulating appetite, are also affected by the seasons. Leptin is secreted by adipose tissue to signal satiety, while ghrelin is secreted by the stomach to stimulate appetite. Shifting sunlight patterns and decreased physical activity can subtly alter their balance. For instance, when physical activity and sunlight exposure decline in winter, leptin resistance, where the brain fails to perceive fullness despite adequate leptin, can increase, or ghrelin secretion may be stimulated. Additionally, metabolism increases to sustain body temperature and brown adipose tissue may activate, requiring more energy and prompting a greater appetite.
In summer, the process reverses. Longer daylight hours suppress melatonin secretion and encourage physical activity, while the body adjusts to lower body temperature and reduce energy intake to protect itself from heat. In hot weather, blood shifts toward the skin to release heat, which relatively reduces blood flow volume to the digestive organs, impairing digestive capacity and leading to a loss of appetite. Furthermore, increased sweating prompts a natural preference for fresh, water-rich fruits and vegetables to replenish fluids and electrolytes. Through these coordinated responses, the body continually adapts to seasonal changes. Recognizing these biological mechanisms allows you to respond thoughtfully to your body's signals rather than attributing seasonal changes to a lack of willpower.
