- an innate mechanism of the body that regulates its periodic cycles or biorhythms, such as the sleep-wake cycle.
- an innate mechanism regulating the passage of an organism through the natural stages of its life, especially with reference to the reproductive stage of a woman's or man's life or the approaching end of this stage: My biological clock is ticking, but I don't feel ready to have kids!
- an inherent periodicity in the physiological processes of living organisms that is not dependent on the periodicity of external factors
- the hypothetical mechanism responsible for this periodicity
