"Day and Night" and "Night and Day" Text/Du Yipin

The idiom "day and night" evolved from "day and night". Originally, only the term "day and night" was used to describe the phenomenon that some things should only be done at night, and some people not only did it at night, but continued to do it during the day. For example, the Cheng Emperor of the Western Han Dynasty and the Zhengde Emperor of the Ming Dynasty were absurd and unscrupulous people, who not only had sexual pleasure at night, but also continued to do so during the day. "Day and night" was originally a derogatory term.

Later, people also saw such a phenomenon, some people worked during the day and continued to work at night. So people turned "day and night" upside down and changed it into "night and day" to describe those who worked day and night. "Night and day" is generally a positive word.