The Entrainment of Circadian Systems

Abstract

The entrainment of circadian systems is essential for their functional significance as well as for our insight into their physiologic organization. Entrainment entails the adjustment of both the frequency and phase of rhythms in the living world to the cycle of the earth's rotation. It is only by virtue of entrainment that programs in behavior and physiology produced by endogenous circadian systems can be properly timed. This is crucial for the advantages in natural selection that in the past gave rise to the evolution and today maintain the genetic basis of these systems. Entrainment requires the sensitivity of endogenous oscillators toward particular environmental cues as well as insensitivity toward others. The sensitivity toward light has been and continues to be a primary guide in probing and unraveling the physiology of circadian systems.