It's an innate periodic cycle in an organism's physiological processes as in sleep & wake cycles
... Noun, 1. biological clock - an innate mechanism in living 
organisms that... [DOC] [TLE] Biological Rhythms as a Basis for Mood Disorders [PAR] The circadian clock relies on environmental cues to regulate its function, ... 
Circadian rhythms are defined as an endogenous rhythm pattern that cycles on a 
... More specifically these physiological and behavioral rhythms control the 
waking/sleep cycle, ...