biological clock

Từ điển Oxford Advanced Learner 8th

ˌbioˌlogical ˈclock [biological clock] BrE NAmE noun (technical)
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.