37. The Principles of Oriental Medicine ： Biological Clock and the Meridian Flow

This year’s Nobel Prize in Physiology or Medicine was awarded to scientists who uncovered the mechanisms of the biological clock. The concept of the biological clock posits that all living organisms perceive light and, based on that, set an internal time. This time then governs various physiological activities to maintain optimal health.

In the East, as far back as 3,600 years ago during the Yin dynasty, people used the heavenly stems and earthly branches (천간지지, Tiāngān Dìzhī) for calendrical purposes, describing the energies and movements of heaven and earth and applying them to daily life. The sexagenary cycle (육십갑자, 60-year cycle) is formed by combining the ten heavenly stems (갑, 을, 병, 정, 무, 기, 경, 신, 임, 계) with the twelve earthly branches (자, 축, 인, 묘, 진, 사, 오, 미, 신, 유, 술, 해), resulting in 60 unique year designations. For instance, this year is Jeongyu-nyeon (정유년), and next year will be Musul-nyeon (무술년).

(As an aside: while not a confirmed historical fact, the Yin dynasty is believed by some to have been established by the Dongyi people. Since Chinese characters were used during that period, some scholars argue that these characters may have originated with the Dongyi. In fact, while practicing in China, I was asked about this theory several times by Chinese scholars.)

More than 2,200 years ago, the classical text Huangdi Neijing (황제내경) applied the heavenly stems and earthly branches to human physiology and pathology, presenting a detailed explanation of biological rhythms, life-nurturing methods (yangsheng), and even treatment techniques. This framework is called Zi Wu Liu Zhu (자오유주, 子午流注).

In this context, Zi (子) symbolizes yin (陰), the essence of life associated with water, and Wu (午) symbolizes yang (陽), associated with fire. These characters represent not only day and night but also summer and winter, south and north—thus encompassing both time and space, yin and yang, life and death.

The theory holds that the body’s organs (zangfu) and meridians (jingluo) operate in rhythm with external environmental factors such as time, season, and direction. For example, between 7 and 9 a.m., the stomach meridian is most active, making it the best time to eat breakfast. Between 9 and 11 a.m., the spleen meridian is most active, aiding in the digestion of consumed food.

Korean medicine views nature as the macrocosm and the human body as the microcosm. Therefore, it proposes specific life-nurturing methods (양생법) in alignment with natural changes throughout the day and across seasons. When using acupuncture and herbal medicine, practitioners consider both external factors like time and season, and internal factors such as the physiological flow of organs and meridians.

Dr. Jin-man Kim, director of Peace Oriental Clinic