Circadian Rhythm - Criteria

Criteria

To be called circadian, a biological rhythm must meet these four general criteria:

1. The rhythms repeat once a day (they have a 24-hour period). In order to keep track of the time of day, a clock must be at the same point at the same time each day, i.e. repeat every 24 hours.
2. The rhythms persist in the absence of external cues (endogenous). The rhythm persists in constant conditions with a period of about 24 hours. The rationale for this criterion is to distinguish circadian rhythms from simple responses to daily external cues. A rhythm cannot be said to be endogenous unless it has been tested in conditions without external periodic input.
3. The rhythms can be adjusted to match the local time (entrainable). The rhythm can be reset by exposure to external stimuli (such as light and heat), a process called entrainment. The rationale for this criterion is to distinguish circadian rhythms from other imaginable endogenous 24-hour rhythms that are immune to resetting by external cues, and hence do not serve the purpose of estimating the local time. Travel across time zones illustrates the ability of the human biological clock to adjust to the local time; a person will usually experience jet lag before entrainment of their circadian clock has brought it into sync with local time.
4. The rhythms maintain circadian periodicity over a range of physiological temperatures; they exhibit temperature compensation. Some organisms live at a broad range of temperatures, and differences in thermal energy will affect the kinetics of all molecular processes in their cell(s). In order to keep track of time, the organism's circadian clock must maintain a roughly 24-hour periodicity despite the changing kinetics, a property known as temperature compensation.