Circadian rhythms

Definition

Endogenous, self-sustaining biological oscillations with a period of approximately 24 hours that synchronize physiological and behavioral processes to daily environmental cycles, primarily light–dark transitions. In arthropods, circadian rhythms govern eclosion timing, locomotor activity, foraging, mating, and metabolic shifts, coordinated by central and peripheral clock genes (e.g., period, timeless, Clock) that maintain rhythmicity even under constant conditions.

Etymology

From Latin circa (about) + dies (day), coined by Franz Halberg in 1959 to describe ~24-hour biological cycles.

Example

In Drosophila melanogaster, eclosion of adults from pupae peaks at dawn due to a circadian clock in the brain that gates emergence to the cooler, more humid morning hours, enhancing survival; the rhythm persists in constant darkness, demonstrating its endogenous nature.

Synonyms

• Circadian cycles
• Daily biological rhythms

Related Terms

• photoperiodism
• Zeitgeber
• Chronobiology
• Diurnal
• Nocturnal
• Crepuscular
• eclosion
• Entrainment
• Free-running rhythm
• Suprachiasmatic nucleus

Usage Notes

Distinguished from ultradian (<24 h) and infradian (>24 h) rhythms. The term implies endogenous origin; externally forced 24-hour patterns without internal clock involvement are technically 'diel' cycles, not circadian. Arthropod researchers often emphasize 'free-running' period length under constant conditions as the diagnostic criterion.