Circadian phase and sex shape swimming exercise responses and stereotyped behaviors in mice

Exercise provides broad health benefits, including improved emotional well-being and cognitive function. Emerging evidence suggests that exercising at different times during the day can have differential effects. However, how circadian phase and sex influence behavioral and physiological responses to exercise remains unclear. To address this question, we examined male and female wild-type mice maintained in either regular (REG, lights on/off at 7AM/7PM) or inverted (INV, lights off/on at 10AM/10PM) light cycles. Mice were then subjected to daily 20-min group swimming exercise sessions at ZT2-3 for 3 weeks. Exercised and sedentary controls mice were then subjected to an open field test (OFT) and blood corticosterone (CORT) measurements 24 hours post-exercise. We quantified several behaviors during swimming: escape attempts, floating, climbing and collisions. We also identified a novel swimming behavior: floating with only nostrils-above-water events (NAWEs). We found that expression of these behaviors was differentially modulated by sex, light-cycle and their interaction. Notably, behavioral differences were more pronounced in REG mice (rest phase). REG mice also lost weight after exercise and had elevated CORT levels compared to mice kept in INV conditions (active phase). Interestingly, OFT behaviors showed significant differences primarily in INV mice, particularly females, when comparing exercised vs sedentary groups. Our novel findings reveal that circadian rhythms and sex significantly interact to shape swimming exercise and stereotyped behaviors in mice. This emphasizes the need to consider the animals circadian phase when designing preclinical studies to match intended behavioral and physiological outcomes. HIGHLIGHTSCircadian phase and sex jointly shape swimming behavior patterns. Newly identified swimming behavior is more prevalent during rest-phase Restphase exercise produced stronger behavioral and physiological effects. Rest-phase exercise resulted in weight loss and elevated stress markers. Active-phase exercised females showed the strongest open field behavioral differences.