Circadian rhythm refers to an internal biological clock that runs approximately on a 24 hour cycle.
It is an innate, daily, fluctuation of behavioral and physiological functions, including sleep waking, generally tied to the 24 hour day-night cycle but sometimes to a different (e.g., 23 or 25 hour) periodicity when light/dark and other time cues are removed.
Other /More definition:
Circadian rhythm refers to the daily biorhythm oscillation of heightened and lowered brain arousal observed throughout the wake/sleep cycle.
It is likewise, the 24-hour cycle of physiological functions maintained by every person; the sleep-wake cycle