In ancient China, through observing the changes in nature, such as the contradiction between earth and sky, day and night , sun and moon, cloudy and clear, cold and hot, water and fire, men and women, etc., people realized the characteristics of “yin–yang” changes and summed them up to form certain theories [1]. At the end of the Western Zhou Dynasty (AD eleventh century–771 BC), the concept of yin–yang was gradually concluded from contradictions in nature. People explained the reason for causing things to change by “yin–yang” changes. Laozi (《老子》) said, though yin and yang was mutually contradictory, they were actually in unity. The contradiction between yin and yang exists everywhere in the world. Yizhuan (《易传》) first put forward that Tao means the combination of yin and yang, which means that yin–yang plays the most important part in Chinese Philosophy . On the one hand, yin–yang emphasizes the differences and opposites between contradiction in movements; on the other hand, it emphasizes the combination and unification in the formation of objects. Yin and yang achieve relative balance in alternations. On the basis of traditional philosophy, TCM developed the theory of circadian rhythm .

Suwen (《素问》), which is one of the most important books in TCM, said yin–yang is the principle of nature, the routine of all things, the source of changes, and the beginning of growth. TCM associates all kinds of human life activities with the natural phenomena and also correlates them to the physiological functions of viscera. Then the rhythm of viscera, diseases, cure, acupuncture, and so on were formed.

According to the theory of TCM, the changes of viscera rhythm follow the changes of yang in nature. The changes of human body follow the alternation of day and night . Suwen–Shengqi Tongtian Lun (《素问·生气通天论》) said “yang qi hosts the exterior in day time, beginning ascending at dawn, being prosperous at noon and being weak at sunset.”

The ancients used 12 Earthly Branches (di-zhi地支) for timing. The 12 Earthly Branches divide a day and a night together into 12 sections, which are also called 12 double-hour (Shichen 时辰). Qi and blood flowed through the 12 regular channels into 5 zang viscera and 6 fu viscera according to certain sequence from 3 a.m. (Yinshi 寅时). When the functions of zang viscera or fu viscera were poured by qi and blood, it reached its functional peak period. So the functional peak period of 5 zang viscera and 6 fu viscera was determined as follows: 11 p.m.–1 a.m. (Zishi 子时) to 1–3 a.m. (Choushi 丑时) for liver and gallbladder, 3–5 a.m. (Yinshi 寅时) to 5–7 a.m. (Moushi 卯时) for lung and large intestine, 7–9 a.m. (Chenshi辰时) to 9–11 a.m. (Sishi 巳时) for stomach and spleen,11 a.m. to –1 p.m. (Wushi午时) to 1–3 p.m. (Weishi 未时) for heart and small intestine,3–5 p.m. (Shenshi申时) to 5–7 p.m. (Youshi 酉时) for bladder and kidney, 7–9 p.m. (Wushi戊时) to 9–11 pm (Haishi亥时) for pericardium and sanjiao.