Yin and Yang (阴阳)
Yin and Yang is a concept originating in Daoism, and Daoism is the root of Chinese medicine.
This site uses mathmatics to explain the development of yin yang theory:
Anyway, Yin implies night, shade, and cold; Yang implies day, warmth, light, activity. So when this is applied to the human body, Yin implies sleep, rest and nourishment, and Yang implies exercise, movement and warmth.
So, in order to be healthy, Yin and Yang should be harmonized. In other words, people should get enough sleep and enough exercise. They should wear more clothes in winter and try to stay cool in summer.
This is all just common sense really. If someone can't sleep, or doesn't exercise, then they aren't healthy.
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