This word is frequently spelled "nycthemeron". It was written as such because in classical times a new day started at sunset, a rule continued in some religions today. Although nychthemeron is generally unknown, it is still used by some people, as well as its adjective, "nychthemeral".

Those scientists who study the ways in which the body's physiological activity varies hour by hour, as in sleep research, make a distinction between nychthemeral and the more common Latin-derived circadian which comes from circa, "round, around" + dies, "day".

Circadian refers to daily cycles that are driven by an internal body clock, while nychthemeral rhythms are the result of the external environment.