Yet, this observable duality is nothing else but the 'side effect' of the incessant cycle from sunrise - sunset, winter-summer and around again with centre the sun.
Ancient texts (i.e."Huangdi Neijing-Sowen" 皇帝內經 -素聞 ) expresses the phenomenon described above as follows:
"When Yang (Sun, light) rises, Yin expands (away from the contraction which is natural to it), when Yang (Sun Light) declines, Yin hides (contracts to its maximum)*."
Thus, when the sun rises, light and warmth come and i.e. the flowers open. When the sun dawns, light 'succumbs to darkness and coldness.' Darkness is absence of sun and light from absence of sun. This is when the moon appears. Sun in Chinese is called "Taiyang 太陽, greatest yang" and the moon Taiyin 太陰, Greatest Yin.
Yang and Yin are nothing else but metonyms of sun's and moon's expressions and behaviour.
In terms of human physiology, the early part of the day gradually leads to activity, whereby people get out of sleep and go about their own affairs 'using energy' and 'connecting' with the world, perceived as 'external' to them. The later part of the day gradually leads to retreating, when people go back to their homes, seek rest and 'connect' with themselves, with a perceived 'inner' world. This is time to replenish and refuel one's power. Yet both moments, despite appearing 'opposite' are one movement, one life, one cycle: one needs to use to live and needs to rest to refuel. both serve the one single life manifest. Incidentally, the latter aspect of life, is associated with transformation 化, which is when whatever has been gathered in the daily connection with the 'external' world (food, experiences, air, sound, emotions, odours, stimulations etc) undergoes a metamorphosis to become 'mine'.
From the point of view of both Chinese philosophers and doctors, the propriety, correctness, wisdom, truthfulness and uprightness of it all rest on the 'logos' beyond all observable phenomena. They call it: Dao, void, the path from which everything proceeds. As common denominator of all life, it also evokes 'oneness' of all living beings.
Observing the 'external' world' of multiple phenomena, it seems that reality is founded upon duality and difference. However, these are superficial, as Dao 'oneness' is not only the fundamental structure of their existence, but also the ultimate abode of their life journey.
Disconnecting and reconnecting happens during the whole of one's lifespan : all life is nothing else, but an exercise of emerging from the void as creator and needing to go back to it to a 'pause' that allows creation to happen over and over again.
All forms always move from within this oneness, this centre; regardless of how the whole world seems broken into bits, the sage never departs from this position of 'oneness'. Thus, he/she moves in the world, without allowing anything to separate him/her from Dao and from resonating with the whole.
Why is this relevant to Chinese medicine?
The 1st Chapter of the "Huangdi Neijing - Suwen 黃帝內經素聞“, stresses how, in order to be healthy we need to make the shift not from worldly to heavenly concerns, but from making a too sharp distinction between "inner" or "outer' directions, but to stay centred in Dao, which is ultimately both.
* 1. For more on this please read: HUangdi Neijing -Sowen Ch 1
* 2. In the evening the sun drops out of sight and it may seem that darkness swallows it.