Definition: A full period of night and day. The plural is either nychthemera or nychthemerons.
From the Greek, “nyktos” for night and “hemera” for day. It was written in that order, with night preceding day, because in Greece it was believed that a new day was started at sunset rather than sunrise. It connotes the changes that take place over a twenty four hour cycle external to the human body, in contrast to the word circadian which refers to the changes over that time span within the body.
Why this word?
I find the contrast that nychthemeron presents to circadian to be quite interesting. I had never given any thought to the idea that circadian rhythms might have a counterpoint or balance but rather thought that they exerted a control all their own. Additionally, I find the usage of one word to describe the unit of day and night together to be rather poetic. In English we are accustomed to the two being divided. There is day, and there is night. Yet here they are married in a rather poetic form in nychthemeron.
How to use the word nychthemeron in a sentence?
“It took a complete nychthemeron before she would even begin to consider the idea, but I finally wore her down.”
“My love, it is but one full nychthemeron until we shall see each other again.”