Real Dictionary

biological clock
Your biological clock is your body's way of registering time. It does not rely on events such as day or night, but on factors such as your habits, your age, and chemical changes taking place in your body.

biological clock in American English
any of the various natural cycles in organisms that are related to the tides, sun, moon, light, temperature, etc. and that control breeding, feeding, migration, etc.
noun: an innate mechanism of the body that regulates its rhythmic and periodic cycles, as that of sleeping and waking

biological clock in British English
noun: an inherent periodicity in the physiological processes of living organisms that is not dependent on the periodicity of external factors