It's an innate periodic cycle in an organism's physiological processes as in sleep & wake cycles
[DOC] [TLE] Biological clock - definition of biological clock by The Free Dictionary [PAR] An internal mechanism in organisms that controls the periodicity of various ... (
Biology) an inherent periodicity in the physiological processes of living 
organisms that is not ... regulates its rhythmic and periodic cycles, as that of 
sleeping and waking.