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

biological clock

noun
 
/ˌbaɪəˌlɒdʒɪkl ˈklɒk/
 
/ˌbaɪəˌlɑːdʒɪkl ˈklɑːk/
(specialist)
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  1. 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.
    • With jet lag, your biological clock is out of synch with the actual time.
See biological clock in the Oxford Advanced American Dictionary
ancient
adjective
 
 
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