Relationship between free-running period and minimum tolerable light pulse interval of skeleton photoperiods in field mice Mus booduga

The entrainment of the circadian rhythm of locomotor activity was studied in the field mouse Mus booduga in order to examine the relationship between the free-running period (τ) and minimum tolerable light pulse interval of the skeleton photoperiods. The animals were entrained under three different light/dark (LD) schedules, each out of phase with the other. They were then subjected to various skeleton photoperiods created by two repeated light pulses (LPs) interrupting darkness. Animals that selected the shorter interval between the LPs as their “subjective night” had significantly shorter τ (23.13 ±0.38h) as compared to those that selected the longer dark interval as subjective night (τ = 23.87 ± 0.18h). When the longer dark interval was 12h, animals selecting that interval as their subjective night included both long-τ and short-τ individuals. When both intervals of darkness were of equal duration, no difference in the selection of subjective night was seen between short and long-τ animals. When the “dusk” LP for the animals that selected the longer dark interval as subjective night was advanced by 2h to create a new skeleton photoperiod, the number of transient cycles appearing before steady-state entrainment was found to depend on the duration of the photoperiods. When the night defined by the two LPs was reduced below 6h, a dramatic “phase jump” in the activity rhythm was observed, and the initial phase relationship was restored after a relaxation in the night duration. We observed considerable interindividual variation in the “minimum tolerable light pulse interval of skeleton photoperiods,” which we suggest may be due to the observed variation in τ among individuals