丁曰。天地交泰。日月晓昏。人之寤寐。皆相合也。少壮未损其荣卫。故寤寐与天地阴阳同度。是以昼日精强夜得
其寐也。老者损瘁。故昼日不能精强。荣卫滞涩。所以夜不得寐也。是以昼日不精而夜不得寐也。杨曰。卫气者、昼日
行于阳。阳者。身体也。夜行于阴。阴者、腹内也。人目开卫气出则寤。入则寐。少壮者。卫气行不失于常。故昼得安
静而夜得稳眠也。老者卫气出入。不得应时。故昼不得安静。夜不得寐也。精者静。静、安也。
四十七难曰。人面独能耐寒者。何也。然。人头者、诸阳之会也。诸阴脉皆至头、胸中而还独诸阳脉皆上至头耳。
故令面耐寒也。
丁曰。天地阴阳升降。各有始终。阳气始于立春。终于立冬。阴气始于立秋。终于立夏。其小满、芒种、夏至、小
暑、大暑。此五节故以法象于头。故面独能耐寒。其小雪、大雪、冬至、小寒。大寒。此五节法象人之足。亦不耐其寒。
此之谓也。杨曰。接诸阴脉皆至颈、胸中而还。盖取诸阳尽会于头面。诸阴至头面者少。故以言之耳。经云。三百六十
五脉。悉会于目。（按灵枢邪气脏腑病形篇云。十二经脉。三百六十五络。其血气皆上于面而走空窍。此所引有脱误。）
如此。则阴阳之脉皆至于面。不独言阳脉自至于头面也。
Ding stated: When Heaven and Earth interact harmoniously, and day and night alternate between light and dark, human wakefulness and sleep are similarly in harmony. In youth and strength, when Ying (Nutritive Qi) and Wei (Defensive Qi) are not depleted, one's cycle of wakefulness and sleep aligns with the rhythm of Yin and Yang in Heaven and Earth. Thus, one is vigorous during the day and sleeps well at night. The elderly, however, suffer from physical decline; their Ying and Wei Qi become stagnant and sluggish, preventing them from being vigorous during the day and unable to sleep at night. This explains why they lack vigor by day and cannot sleep by night.
Yang stated: Wei Qi (Defensive Qi) circulates in the Yang aspect during the day, where "Yang" refers to the exterior body surface; it circulates in the Yin aspect at night, where "Yin" refers to the interior abdomen. When a person opens their eyes, Wei Qi exits the exterior, resulting in wakefulness; when it enters the interior, it results in sleep. In the young and strong, Wei Qi circulates without deviation from its normal pattern, allowing for calmness during the day and steady sleep at night. In the elderly, the entry and exit of Wei Qi do not correspond to the proper times, leading to restlessness during the day and inability to sleep at night. Essence implies stillness; stillness implies tranquility.
Difficulty 47 asks: Why is it that only the face can resist cold? The answer is: The head is the convergence point of all Yang meridians. All Yin meridians reach the head and chest before returning, whereas all Yang meridians ascend to the head and ears. Therefore, the face is able to resist cold.
Ding stated: The ascent and descent of Yin and Yang in Heaven and Earth have their beginnings and ends. Yang Qi begins at Lichun (Start of Spring) and ends at Lidong (Start of Winter); Yin Qi begins at Liqiu (Start of Autumn) and ends at Lixia (Start of Summer). The five solar terms Xiaoman (Grain Buds), Mangzhong (Grain in Ear), Xiazhi (Summer Solstice), Xiaoshu (Minor Heat), and Dashu (Major Heat) correspond symbolically to the head; thus, the face alone can resist cold. Conversely, the five solar terms Xiaoxue (Minor Snow), Daxue (Major Snow), Dongzhi (Winter Solstice), Xiaohan (Minor Cold), and Dahan (Major Cold) correspond symbolically to the feet; the feet also cannot resist cold. This is what is meant.
Yang stated: The connecting Yin meridians all reach the neck and chest before returning. This is likely based on the principle that all Yang meridians converge on the head and face, while few Yin meridians reach the head and face, hence this explanation. The Classics state: "The three hundred and sixty-five vessels all converge at the eyes." (Note: According to the chapter "Pathogenic Factors and Zang-Fu Diseases" in *Lingshu*, "The twelve meridians and three hundred and sixty-five collaterals, their blood and Qi all ascend to the face and enter the orifices." The citation here contains omissions and errors.) If this were true, then both Yin and Yang meridians would reach the face, not just the Yang meridians ascending to the head and face.