What is an endogenous clock? How are circadian rhythms distinguished from photoperiodic behavior?
The term endogenous refers to the rhythms internally generated by an organism. Rhythms related to environmental periodicity should disappear under controlled, continuous darkness. Conversely, circadian rhythms, by definition, cannot be terminated but can be phase-shifted. Special Topic Report: The Importance of Light in the Circadian Cycling of Vertebrate Systems (by Allison Konefal) The manifestations of distinct internal regulatory mechanisms can be observed to varying degrees amongst vertebrates. Those biological rhythms which typically manifest a 24 hour cycling period are known as circadian rhythms. Set apart from other physiological variables characterized by cyclic patterns, circadian rhythms involve very specific relationships between zeitgebers (stimuli such as light, temperature, food availability, etc.) and the activation of sensory responses. One particularly significant correlation exists between the presence of light and the body s physiological conditioning. By virtue of
