Spot the correct spelling of a 24-hour biological cycle.

Correct — B. Circadian is built from Latin circa (about) and dies (day): 'about a day', which is why it names a roughly 24-hour cycle.

The day root keeps its di, followed by the adjective ending -an: circa + di + an. Meridian is built the same way, from meridies, midday.

That spells Circadian → option (b).

Why the others are wrong

• (a)
  Circadion — Circadion ends in -ion, the noun ending of words like region and opinion. Circadian is an adjective, and after the di of dies it takes -an.
• (c)
  Circadien — Circadien uses -ien, an ending that looks French. The Latin-built word keeps the a: circa + di + an.
• (d)
  Circadijan — Circadijan inserts a j that has no source. Neither circa nor dies contains a j, so none can appear in the word built from them.

A circadian rhythm is the body's internal cycle of roughly 24 hours, which governs sleep and waking, among other things.

The 'roughly' is in the word itself: circa means about, as in circa 1850. Without daylight cues, the cycle runs close to 24 hours, not exactly 24.

Spelling it from its parts, circa, di(es) and -an, makes the three wrong options easy to reject.

Key facts

• Circadian comes from Latin circa (about) and dies (day).
• A circadian rhythm is an internal cycle of about 24 hours that regulates sleep and waking.
• Meridian also carries dies, through meridies, midday.

Common traps

• Spelling the unstressed ending by ear and writing -ion or -ien.
• Taking circadian to mean exactly 24 hours, when circa means about.

No directly related past PYQ was found.