That ties in with concepts of Chinese Traditional Medicine. The Yin (female) meridians are always working - spleen, lung, kidney, liver, heart, circulatory system. Whereas the Yang (male) meridians work then rest - stomach, large intestine, small intestine, gall bladder, bladder, triple warmer. Historically, biologically, women would have been working / on alert with a wide area of focus, whereas men would have worked very hard then rested.