biological clock

Compound Noun (Abstract): ""Biological clock" is an abstract compound noun because it refers to an internal, naturally occurring system in organisms that regulates cycles of activity, such as sleep, fertility, or aging. 

It is countable when referring to specific systems or metaphors:

“Her biological clock was a factor in her decision to have children.”

It is a compound noun, formed from the adjective biological + the concrete noun clock, creating a metaphorical phrase.

• The natural system that controls when you wake, sleep, eat, or feel alert (circadian rhythm).
• A metaphorical term used to describe a woman’s fertility timeline, especially the perceived urgency to have children before a certain age.
• Any internal timing mechanism in organisms governing periodic biological activities.

• Your biological clock adjusts gradually to changes in time zones.
• She felt the pressure of her biological clock ticking in her mid-30s.
• Animals rely on their biological clocks to migrate or hibernate seasonally.