circadian \sehr-KEYD-(i-)yehn\ (adjective) - Pertaining to a 24-hour day, especially, occurring every 24 hours.
"Rob's circadian rhythm corresponded more to the west coast than the east, making him more than a bit of a night owl."
Today's word is made up of Latin circa "around" + dies "day." The original form of "dies" was Proto-Indo-European *dyeu- "Jove," the Indo-European god of the bright sky. The name "Jove" (whence "jovial") is a descendant of this root, as is "Jupiter," originally a compound of *dyeu + *peter "father," i.e. father of Jove. The name of Diana, the Roman goddess of the moon, share the same origin, as does the name of the supreme Greek deity, Zeus. The same may be said of the word "deity" and most of the words borrowed from Latin referring to days, such as "diurnal," "diary," "journey," and "journal," the last two of which come to us via French journee "day"