Circadian rhythm is biological diurnal (daily) rhythms of living organisms. It comprises behaviour and processes in the body that are regularly repeated throughout a c.24 hour cycle.
The circadian rhythms are part of our biological clocks.
Circadian rythm in humans. Photo: Yassine Mrabet, CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0), via Wikimedia Commons)