The classical model for circadian entrainment (Pittendrigh 1981) postulates that the endogenous circadian cycle length (?) deviates slightly from 24 h (T) in absence of a Zeitgeber. During entrainment, a daily phase shift equal to the difference between ? and T corrects for this deviation. This is considered to be a biological imperfection, solved by natural selection through the phase-dependent sensitivity needed anyway for maintaining a specific phase relationship.
Recently, Beersma et al. (1999) proposed a modified concept for photic entrainment in which a continuous response to light is of dual nature: not only the phase is reset, but also the period itself is adjusted. Under constant conditions, the period subsequently only gradually reverts to its long-term value.
This concept has several features extending insight into circadian organisation beyond the classical model:
- Under conditions of entrainment, the instantaneous value ? is valid instead of the long- term value in DD
- The model renders specific meaning to after-effects.
- The model allows entrainment for animals not seeing the twilights.
- The model allows entrainment in highly irregular and imprecise Zeitgebers, and integration of daily light profile information.
By period adjustment, the system acts as a long-term integrator of daily light profile information. This is a learning performance, and may give hints towards the role of circadian pacemakers in learning and memory.
The project's challenge is to enlarge upon the theoretical analysis of the model, to test a number of predictions, in particular in a comparative analysis of diurnal and nocturnal mammals. Questions will be tested in experiments employing a newly developed computer-controlled illumination system for small mammals. Appropriate simulations will be used in parallel. |
