The Entrainment of Circadian Oscillations by Light and Their Role as Photoperiodic Clocks

Bünning's general proposition, that circadian rhythmicity underlies the photoperiodic time-measurement, is, in our view, correct. In the first place the proposition seems highly plausible, a priori, in view of the diversity of other chronometric functions such rhythms subserve. In the second place there is a large body of experimental fact that cannot reasonably be interpreted in any other way. A "Coincidence Model" for photoperiodic induction is outlined; it is essentially Bünning's original scheme given in somewhat more explicit terms. It may yet prove true that the "coincidence-device" type of model will prove inadequate; but that would not necessarily render Bünning's more general proposition invalid-that the circadian system somehow executes the time-measurement. In any event we note that any general theory of circadian oscillations as photoperiodic clocks must go well beyond the terms in which the proposition was first stated; and specifically it must incorporate a general theory of the entrainment of such oscillations by light. Classical photoperiodic induction is, in our view, only an extreme case of the general seasonal modulation of the state of the circadian system that photoperiod (as the entraining agent) effects. The theory of entrainment outlined here gives an explicit basis for interpreting the action of night interruptions; and, among other things, indicates that the concept of "scotophil" needs more rigorous definition. As a specific fraction of the circadian cycle, it must be defined in terms of SCT time. Its discussion in terms of AZT time (which is common in the literature) leads to a confounding of two distinct functions of the light: entrainment and induction. The most direct and unequivocal test of any theory involving the coincidence of a specific fraction of the circadian cycle with light is offered by the theory of entrainment for short pulses. Entrainment to T values close to τ involves fully predictable coincidence, or non-coincidence, of the short light pulse with particular SCT phases. The one such test so far performed supports the coincidence model. The possibility of two distinct pigments underlying the two distinct functions of light is raised.