Aschoff's Rules

Aschoff's Rules consist of three generalized statements that were first introduced by Jürgen Aschoff. These rules are fundamental in the field of chronobiology as they describe how the circadian rhythms of both diurnal and nocturnal animals are impacted by varying light conditions.

The circadian rhythm, regulated by a circadian pacemaker, demonstrates endogenous and entrainable oscillation with a period close to 24 hours. The time required for one circadian oscillation to occur under constant conditions is known as the free-running period (τ). In a natural setting, the circadian rhythm is synchronized with the external environment through entrainment to Zeitgeber signals. Based on his observations of the free-running periods (spontaneous frequencies) of several animal species in varying light exposure, Aschoff proposed Aschoff's rules in collaboration with Colin Pittendrigh, demonstrating how duration and intensity of light exposure affect the circadian pacemaker and behaviors of animals.

Aschoff's Third Rule is the most referenced one because it directly relates changes in circadian period as the result of light intensity. Research on several mammalian species supports Aschoff's rules by lesioning anatomical features related to the circadian clock, demonstrating how circadian rhythm is regulated. Although these rules have remained significant, multiple exceptions have been reported across taxa, especially in arthropods and certain mammalian species. These violations suggest that species-specific adaptations and ecological factors can modulate circadian responses to light. Further studies have also found that Aschoff's First Rule expression is regulated by circadian genes, and mutation of relevant genes causes Aschoff's rule violation.