Definitions from The American College Dictionary

circadian adj. Biology.

Relating to or exhibiting approximately 24-hour periodicity. [Latin circa, around; see CIRCA + Latin dies, day; see deiw- below.]

deiw-. Important derivatives are: Tuesday, deity, divine, jovial, July, Jupiter, Zeus, dial, diary, dismal, journey, psychedelic. deiw-. To shine (and in many derivatives, "sky, heaven, god").
I. Noun *deiwos, god.
1.a. TIU, (TUESDAY), from Old English Thw (genitive Thwes), god of war and sky; b. TYR, from Old Norse Tùr, sky god. Both a and b from Germanic *Thwaz.
2. DEISM, DEITY, DEUS, JOSS; ADIEU, DEIFIC, from Latin deus, god.
3. DIVA, DIVINE, from Latin dhvus, divine, god.
4. DIVES, from Latin dhves, rich (< "fortunate, blessed, divine").
5. Suffixed zero-grade form *diw-yo-, heavenly. DIANA, from Latin DiEna, moon goddess.
6. DEVI; DEODAR, DEVANAGARI, from Sanskrit de- vaµ, god, and deva-, divine.
II. Variant *dyeu-, Jove, the name of the god of the bright sky, head of the Indo-European pantheon.
1. JOVE, JOVIAL, from Latin Iovis, Jupiter, or Iov-, stem of Iuppiter, Jupiter.
2. JULY, from Latin Iulius, "descended from Jupiter" (name of a Roman gens), from derivative *iou-il-.
3. Vocative compound *dyeu-põter-, "O father Jove" (*põter-, father; see põter-). JUPITER, from Latin Iuppiter, Iupiter, head of the Roman pantheon.
4. DIONE, ZEUS; DIOSCURI, from Greek Zeus (genitive Dios), Zeus.
III. Variant *dyT- (< *dyeõ-). DIAL, DIARY, DIET2, DISMAL, DIURNAL; ADJOURN, CIRCADIAN, (JOURNAL), (JOURNEY), MERIDIAN, (POSTMERIDIAN), QUOTIDIAN, SOJOURN, from Latin diTs, day.
IV. Variant *deiõ-. PSYCHEDELIC, from Greek dTlos, (< *deyalos), clear. [Pokorny 1. dei- 183.]