Circadian rhythms are biological processes that occur in a daily, yearly, or seasonal pattern amongst organisms. The Latin root "circa" means "around". The Latin root "diem" means "day". So the word literally means "around the day".
These rhythms are controlled by an internal circadian clock that is controlled by approximately 20,000 nerve cells that are located in the hypothalamus, an area of the brain that resides just above the optic nerves. They have been observed in plants, animals, fungi and cyanobacteria. Examples of circadian rhythms are the sleep/wake cycle, hormonal releases, and body temperature. Although circadian rhythms are largely endogenous, they can be affected by external environmental changes. An example of such would be the jet lag experienced by travelers that occurs because the time that the clock reads at the new location differs from the time of their home location.