Circadian means the biological rhythm over a period of approximately 24 hours. The term comes from the Latin circa (almost) and dies (days).
These biological rhythms are present in all living beings including bacteria and result in cyclical changes in a large number of physiological indices (metabolic activity, hormone secretion, etc.)
The term internal clock is used to describe the effects which give rise to these rhythms and which may also be linked to the environment. The circadian rhythm persists in the absence of sunlight but may shift gradually if it is slightly more or less than 24 hours.
It is known that daily phenomenon such as sunlight contribute to the shifting of the circadian rhythm.