circadian (adj) – of or pertaining to a 24-hour period
BREAKDOWN: CIRC- (around) + DI- (day) + -IAN (of or pertaining to)
— circadian rhythms are physical, mental and behavioral changes that follow an approximate 24-hour cycle, responding primarily to light and darkness in an organism’s environment.
This word is built on the root CIRC-, which means about, around, or round.
Common CIRC- words include circle, circumference, and circus.
More esoteric CIRC- words include circumlocution, circumnavigate, and circumvolve.