Ultradian and circadian rhythms: Experiments and models

Abstract

The importance of both ultradian and circadian rhythms in Nature is well known. In the literature it is possible to find many articles referring to the characteristics and properties of such rhythms and, in some cases, of their mathematical modeling. However, as far as we know, there is not enough information relating these two types of rhythms either from a biological or mathematical point of view. In this chapter we describe the main properties of the crayfish circadian rhythm structure and its mathematical simulation through different developmental stages and experimental situations. During the work we revealed the persistence of the ultradian rhythms even when they are masked by circadian rhythms. Moreover, when transients were analyzed in both models, biological and mathematical, we were led to consider the hypothesis that ultradian rhythms could represent a regression to a primordial vital dynamic state. Finally, we report on certain new experiments and their mathematical models, where we can observe the initial presence of a circadian rhythm and its subsequent regression to again an ultradian one by the influence of an external disturbance. It seems that ultradian rhythms arise first in evolution and in ontogeny, preceding in both cases the appearance of circadian rhythms. © 2008 Springer Science+Business Media B.V.