Definition of biological clock noun from the Oxford Advanced Learner's Dictionary

 

biological clock

 noun
noun
BrE
 
; NAmE
 
(specialist)
 
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a natural system in living things that controls regular physical activities such as sleeping (figurative) At 35, Kate's biological clock was ticking (= she was beginning to think that she would soon be too old to have children). The movements of the leaves are triggered by the plant’s biological clock.
See the Oxford Advanced American Dictionary entry: biological clock