Circadian rhythms are the fluctuations in mental and physical activities of an organism that occur at approximately 24-hour intervals.

 They exist in even the lowliest one-celled plants and animals and are generally controlled by the body’s biological clock, the suprachiasmatic nucleus (SCN) (Hobson 1989).

 Our circadian rhythm is primarily internally controlled, however we often utilize external cues, such as alarm clocks or sunrise and sunset, to plan our sleep/wake cycle. These cues are known as zeitgebers ("time givers"). 

Jürgen Aschoff performed a famous study to determine that the cir cadian rhythm is controlled internally:

In his study, Aschoff isolated human subjects in an underground bunker for a month and allowed them to sleep when they wanted, with the one stipulation that they limit their sleep to one bout per day and did not nap. They were allowed to turn lights on and off as they wished and had no access to zeitbergers. Results of the study showed that subjects’ body rhythms approximated 24-hours, with the average around 25 hours, proving the circadian rhythm is internally regulated (Hobson 1989).

 

 Interestingly, people who are completely blind have a sort of permanent jet lag because they cannot detect light and thus run on the body’s 25-hour cycle in the absence of light cues (ninds.nih.gov).