"Circadian" derived from the word "circle," and refers to your body's rhythm of waking and sleeping in synchronization with the sun rising and setting.
A cycle or rhythm that is roughly 24 hours long; they cyclical daily functions in biological and psychological processes. Like mental alertness and sleep-wake cycle.
It is the bodies life cycle. A daily cycle that the body is used to doing, such as getting up at a certain time, eating at a certain time, sleeping at a certain time and so on.