What is "circadian exhibiting 24-hour periodicity"

Cyclic \Cyc"lic\ (s?k"l?k or s?"kl?k), Cyclical \Cyc"lic*al\ (s?k"l?-kal), a. [Cf. F. cycluque, Gr. kykliko`s, fr. ky`klos See Cycle.]

1. Of or pertaining to a cycle or circle; moving in cycles; as, cyclical time.
   --Coleridge.

2. (Chemistry) Having atoms bonded to form a ring structure. Opposite of acyclic.

   Note: Used most commonly in respect to organic compounds.

   Note: [Narrower terms: bicyclic; heterocyclic; homocyclic, isocyclic]

   Syn: closed-chain, closed-ring.

3. Recurring in cycles[2]; having a pattern that repeats at approximately equal intervals; periodic. Opposite of noncyclic.

   Note: [Narrower terms: alternate(prenominal), alternating(prenominal); alternate(prenominal), every other(prenominal), every second(prenominal); alternating(prenominal), oscillating(prenominal); biyearly; circadian exhibiting 24-hour periodicity); circular; daily, diurnal; fortnightly, biweekly; hourly; midweek, midweekly; seasonal; semestral, semestrial; semiannual, biannual, biyearly; semiweekly, biweekly; weekly; annual, yearly; biennial; bimonthly, bimestrial; half-hourly; half-yearly; monthly; tertian, alternate(prenominal); triennial]

4. Marked by repeated cycles[2].

   Cyclic chorus, the chorus which performed the songs and dances of the dithyrambic odes at Athens, dancing round the altar of Bacchus in a circle.

   Cyclic poets, certain epic poets who followed Homer, and wrote merely on the Trojan war and its heroes; -- so called because keeping within the circle of a single subject. Also, any series or coterie of poets writing on one subject.
   --Milman.