Biological clock

Definition

An internal, self-sustaining physiological mechanism that generates rhythmic biological processes with a period of approximately 24 hours (circadian) or other regular intervals, entrained by environmental cues such as light and temperature but capable of free-running in constant conditions. In arthropods, the clock coordinates daily activity patterns, ecdysis timing, metabolic cycles, and seasonal behaviors such as diapause induction.

Etymology

From Greek bios (life) + Latin cloca (bell), modeled after mechanical timekeeping devices; entered biological usage in mid-20th century with discovery of endogenous circadian oscillators.

Example

The fruit fly Drosophila melanogaster uses a molecular biological clock—conserved across insects and mammals—in which the proteins PERIOD and TIMELESS accumulate during the night and degrade at dawn, driving daily rhythms of locomotor activity and eclosion.

Synonyms

• internal clock
• circadian clock
• endogenous oscillator

Related Terms

• Circadian rhythm
• entrainment
• zeitgeber
• Diapause
• Ecdysis
• Photoperiodism
• chronobiology
• molecular clock (phylogenetics)

Usage Notes

Distinguish from the evolutionary 'molecular clock' used to date phylogenetic divergences. In entomology, 'biological clock' typically refers to the circadian pacemaker, though the term can encompass shorter (ultradian) or longer (infradian, seasonal) rhythms. The clock is said to be 'entrained' when synchronized to environmental cycles and 'free-running' when expressing its intrinsic period (often slightly different from 24 hours) in constant conditions.