The term “12 Two-hour periods (Shí Chen)” first appeared in the Chapter "Yi Feng Zhuan" (Biography of Yi Feng) of the Book of Han (Hàn Shū, 30 C.E.). Based on the twelve sections of the sky that the sun passes through in a 24-hour period from the vantage point of the Arctic, a day was divided into 12 periods which were named after the “terrestrial branchs (Dì Zhī)”. 

To be specific, it refers to periods called Zi, Chou, Yin, Mao, Chen, Si, Wu, Wei, Shen, You, Xu and Hai, corresponding to the process of growth and decline of yin qi and yang qi in a day in nature.

More than two thousand years ago, the medical book The Yellow Emperor's Inner Classic (Huáng Dì Nèi Jīng) described the close relationship between health preservation and the 12 two-hour periods of the day, asserting that each two-hour period corresponds to one of the 12 channels of human body. Since channels are linked with their corresponding zang-fu organs, health preservation based on the 12 two-hour periods helps maintain the corresponding zang-fu organs and thereby strengthen the body.