Modeling the action of zeitgebers on the human circadian system: comparisons of simulations and data.

In our two-oscillator model for the human circadian timing system, the effect of an environmental synchronizing cycle (zeitgeber) is determined by the periods of the two oscillators and of the zeitgeber and by the zeitgeber strength. The oscillators x and y are postulated to regulate the core temperature and rest-activity rhythms, respectively. From published examples of experiments in which human subjects were exposed to artificial zeitgebers, it is possible to derive estimates of the periods of the core temperature and/or the rest-activity rhythms and the zeitgeber. Two strengths of zeitgeber input to the model that correspond to two of the common zeitgeber regimes used in human entrainment studies have now been elucidated. Thus specific model simulations can be generated for each of the available examples of entrainment of human subjects by artificial zeitgebers. Such simulations indicate that the model can reproduce, with remarkable subtlety, the types of full and partial entrainment observed experimentally. The model is thus demonstrated to accommodate the period and phase control of endogenous rhythms by environmental zeitgebers, which is a crucial functional attribute of circadian timing systems.