The word "夜以继日" is formed by sequentially combining the characters for "night" (夜), the preposition "with" or "by means of" (以), the verb "to continue" (继), and "day" (日) in a classical Chinese syntactic structure where "以" connects "夜" as the means to the action "继日", literally meaning "using the night to continue the day", thus creating an idiom that denotes working continuously from day into night.