Our life is regulated by the passage of time, and in particular by the alternance of day and night. This rhythm of about 24 hours, that we take for granted, does control the life of many living beings, including animals and plants, and it is also called circadian rhythm. Circadian comes from Latin and it means “about” (circa) “a day” (diem), that’s because the rhythm in discussion is not always precisely 24 hours but it might vary a bit.
To be more precise, the term circadian rhythm needs to be used only in reference to phenomenon that originate from within the organism (endogenous). In fact, the living beings that follow the circadian rhythm do that also in the absence of external stimuli such as (sun) light or temperature.
Simplifying we could say that many living beings have an internal clock of about 24 hours that regulates the physiological processes and biological activities (sleeping, eating, hormone production, cell regeneration…).