Circadian
Etymology
Coined in 1959 by the scientist Franz Halberg from Latin circa, "around," and dies, "day" — literally "about a day." The "about" matters: the internal clock runs close to, but not exactly, 24 hours, so daylight resets it each morning. The circadian rhythm coordinates sleep, hormone release, and body temperature on that daily loop.
adjective · /sur-KAY-dee-un/
Relating to the roughly 24-hour internal cycle that governs sleep, alertness, and other bodily rhythms.
What it names
Why jet lag and late shifts leave you wrecked — an internal clock, not just fatigue, decides when your body expects to sleep.
Example
Working night shifts scrambled his circadian rhythm, leaving him wide awake at 4 a.m.
Related idea
Cortisol rises and falls on a circadian schedule, peaking in the morning to help you wake.
See also