biological clock

biological clock (plural biological clocks)

1. (biology) Any of several mechanisms that regulate the periodicity of biological functions, such as the circadian rhythm.
   • 2005 US Department of Health - Your guide to healthy sleep

     Your biological clock is actually a tiny bundle of cells in your brain that responds to light signals received through your eyes.

2. (figuratively) A notional clock to which people should pay attention, lest menopause or andropause kicks in and it is too late for them to have children.

   Her biological clock is ticking.

   • 1994, Rugh Winter, Bonnie Eaker-Weil, Adultery, the Forgivable Sin:

     Take Linda, a thirty-nine-year-old newscaster who relished her career but began to hear the alarm ringing on her biological clock.

• body clock

• tick-tock biological clock

• biorhythm
• circadian rhythm