Circadian Shifts Alter Wheel-Running Patterns in Mice Regardless of Estrogen Levels

Introduction: Physical activity is an essential component for good physical health, but there is a physical inactivity problem present throughout the world. Physical inactivity poses potential health problems. Two factors that affect physical activity levels are estrogen and circadian rhythmicity. Additionally, estrogen regulates elements of the circadian rhythms, however, it is uncertain if the relationship between estrogen and the circadian rhythm influences the ability to perform adequate physical activity. The purpose of this study was to evaluate the connection between estrogen levels and circadian shifts on the physical activity levels of female C57BL/6J mice. It was hypothesized that estrogen-deficient mice would have lower physical activity levels and slower re-entrainment rates than estrogen-intact mice after the introduction of circadian shifts. Methods: Eleven mice were separated into 2 groups where one group received ovariectomies to lower their estrogen levels and the other received simulated surgeries where their ovaries stayed intact. Mice were confined in a controlled lighting environment on a 12-hour light/dark cycle. Circadian shifts occurred through adjusting the dark running-period 6 hours for a 7-day period and then an additional 6-hours (12 hours total) for another 7-day period. To measure physical activity levels, wheel running distance, duration, speed, onset, stoppage, and hour-by-hour wheel running patterns were quantified by a turn-by-turn running wheel monitoring system. Results: Wheel running distance (km) [F 1,7 =4.35, p=0.013] and duration (min) [F 1,7 =4.12, p=0.016] were significantly affected by estrogen levels and the introduction of circadian shifts. The onset (hours before or after the beginning of the dark period) of wheel running was significantly affected by the circadian shifts [F 1,7 =42.663, p=0.001], while the stoppage (hours before or after the ending of the dark period) was not significantly affected by the circadian shifts or estrogen levels. Lastly, both ovariectomy and the introduction of circadian shifts changed the hour-by-hour wheel running patterns [F 1,7 =1.78, p=0.007]. In general, ovariectomized mice ran less during each hour of the dark period compared to estrogen-intact mice across all phases of the study. Following the circadian shifts, wheel running in both estrogen-intact mice and ovariectomized mice was suppressed during the first hours (up to approximately four hours after the lights turned off) of the dark period, but it then reinvigorated during the middle and later hours of the dark period. Conclusion: These results suggest that shifting the circadian cues affect the physical activity patterns in mice regardless of estrogen levels resulting in lower levels of physical activity. These effects appear to be driven by when the mice chose to run during the dark period resulting in blunted use of the dark period for running. After shifting the lighting period, mice tended to run during a much smaller part of the dark period showing an inhibited ability to quickly re-entrain to the new circadian cues. Funding for this research was provided by the Union University Department of Biology. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.