Circadian Waveform and Its Significance for Clock Organization and Plasticity

The daily rotation of the earth creates a strong selection pressure for the evolution of endogenous circadian clocks that, at least in mammals, are generally phase shifted slowly and incrementally by light. Because the earth’s axis of rotation is tilted relative to the revolution around the sun, there is an additional selection pressure for clocks to adjust their waveform (i.e., shape of the daily oscillation) to match seasonal variation in daylength. With a focus on rodents, this chapter reviews protocols demonstrating circadian waveform plasticity and its relationship to the functional organization of the suprachiasmatic nuclei (SCN) of the anterior hypothalamus. Manipulation of waveform uncovers additional novel and unanticipated effects on the lability of circadian timing systems.