《黄帝内经·素问·藏气法时论》
肾属水，旺于冬，肾与膀胱互为表里，冬天是足少阴肾经与足太阳膀胱经主治的时间。
肾主冬，足少阴、太阳主治。
Kidney belongs to water and is at its peak in winter; the kidney and the bladder are related as interior and exterior, so winter is the time when the Foot Shaoyin Kidney Channel and the Foot Yangming Bladder Channel are responsible for treatment.