Compressed naturalistic seasonal lighting stabilizes estrous cyclicity in C57BL/6J mice

Photoperiod is a major environmental cue for reproductive timing in mammals, yet laboratory mice are often treated as weakly photoperiodic. Here we tested whether a compressed seasonal lighting paradigm with gradual dawn- and dusk-like transitions shifts estrous cyclicity toward a canonical 4-day pattern in female C57BL/6J mice. From 8 weeks of age, mice were exposed either to progressive day-length extension (8L:16D to 16L:8D) or to the reverse schedule, with the light phase changed by 1 h every 10 days. Within each light period, illuminance and correlated color temperature were changed stepwise to mimic sunrise and sunset. Wheel-running rhythms entrained stably to both lighting schedules. In the short-to-long protocol, total daily wheel-running activity gradually decreased across photoperiods, whereas the inferred estrous cycle length progressively approached the canonical 4-day pattern; when the light phase exceeded 13 h, most animals displayed 4-day cycles, and at 14L:10D nearly all animals did so. In the long-to-short protocol, the 4-day pattern established under long days was largely retained during subsequent day shortening, despite a progressive decrease in total activity. These findings show that female C57BL/6J mice retain substantial photoperiodic sensitivity in reproductive timing under a naturalistic, compressed seasonal lighting schedule. Importantly, the results suggest that estrous cycle periodicity is influenced by the integrated lighting environment and prior photoperiodic history. Because the present design did not isolate the effects of photoperiod length and twilight-like transitions, and because cycle timing was inferred behaviorally from wheel-running profiles, these findings should be interpreted with appropriate caution. Nevertheless, this paradigm may be useful for studying photoperiodic and circadian influences on female reproductive timing under laboratory conditions.