Period responses to Zeitgeber signals stabilize circadian clocks during entrainment

Circadian clocks with characteristic period (tau) can be entrained to light/dark (LD) cycles by means of(i) phase shifts which are due to D/L "dawn" and/or L/D "dusk" transitions, (ii) period changes associated with long-term light exposure, or (iii) by combinations of the above possibilities. Based on stability analysis of a model circadian clock it was predicted that nocturnal burrowing mammals would benefit less from period responses than their diurnal counterparts. The model further predicted that maximal stability of circadian clock is reached when the clock slightly changes both its phase and period in response to light stimuli. Analyses of empirical phase response curve (PRC) and period response curve (tauRC) of some diurnal and nocturnal mammals revealed that PRCs of both diurnal and nocturnal mammals have similar waveform while tauRCs of nocturnal mammals are of smaller amplitude than those of diurnal mammals. The shape of the tauRC also changes with age and with increasing strength of light stimuli. During erratic fluctuations in light intensity under different weather conditions, the stability of phase of entrainment of circadian clocks appears to be achieved by an interplay between phase and period responses and the strength of light stimuli.