Relationship between phase resetting and the free-running period in Djungarian hamsters.

A data set of 293 phase shifts was analyzed in order to determine the relationship between phase resetting and the free-running period (tau) in Djungarian hamsters. Phase shifts in response to a 15-min light pulse were assigned to one of two groups (tau(short), < 24 hr; tau(long), > 24 hr), and two phase response curves (PRCs) were constructed. The two PRCs differed predominantly in the advance region, which extended so far into the subjective day of PRC(long) that a dead zone was lacking. The functional significance of PRC differences was assessed by computer simulations of entrainment to varying skeleton photoperiods and entrainment to a 12-hr skeleton photoperiod with varying tau's. Results from these simulations confirmed the theoretical predictions by Pittendrigh and Daan: Stability of entrainment under varying photoperiods depended on the ratio of the PRC slopes at the phases illuminated by light (S(E)/S(M)). This ratio was always larger than 1 for PRC(long). It approached 0 for PRC(short) as soon as the evening light illuminated the dead zone; this occurred for entrainment to very short photoperiods. Stability of entrainment to lights-off was in general better for PRC(long) than for PRC(short), especially if PRC(long) was used in combination with tau(long). This suggests that it can be advantageous for stability of entrainment to lights-off to express a tau > 24 hr in combination with a PRC lacking a dead zone. Stability of entrainment under varying tau's was not much different for PRC(long) or PRC(short). However, stability of entrainment deteriorated for PRC(long) in combination with short tau's, whereas it deteriorated for PRC(short) in combination with long tau's.