General Features of Entrainment: the Type Model

Abstract

Most experiments designed to examine the synchronization of circadian rhythms have used square-wave light-dark regimes, in which lighting is simply switched on and off at regular intervals. The obvious advantage of this method is its convenience; providing intervening twilights, so as to mimic natural illumination, requires more expensive equipment. An animal’s responses to a square-wave light regime might conceivably differ in important ways from those to natural light regimes, but much experimental evidence indicates that simple on-off light cycles are at least adequate stimuli to synchronize circadian wake-sleep rhythms. Regardless of whether this process resembles what happens in nature, such observations must be accounted for by any satisfactory model. In this chapter we will therefore first ask how the type model responds to simulated on-off light-dark cycles; gradual changes in intensity will be treated later in the chapter.