Time-Dependent Endurance Exercise Improves Metabolic Health Through Circadian Rhythm Regulation in Mice

Objectives: Circadian rhythms regulate key physiological processes, including metabolism and energy balance. Emerging evidence suggests that the timing of physical activity may influence metabolic outcomes. However, how the timing of endurance exercise impacts long-term metabolic health and the role of the circadian clock in this process remain unclear. This study aimed to investigate whether time-dependent endurance exercise improves metabolic health via circadian rhythm regulation. Methods: A 12-week endurance exercise protocol was established using wild-type (WT) and circadian-disrupted Clock^Δ19 mice. Mice were assigned to exercise at Zeitgeber time 0 (ZT0) or Zeitgeber time 0 (ZT12), or to sedentary controls. Assessments included rotarod fatigue test, body weight, epididymal fat ratio, fasting blood glucose, serum triglycerides, high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), non-esterified fatty acids (NEFA), intraperitoneal glucose tolerance test (IPGTT), and insulin tolerance test (ITT). Results: Clock^Δ19 mice exhibited circadian phase-dependent fatigue susceptibility on the rotarod, particularly at ZT0. Both exercised Clock^Δ19 groups (ZT0 and ZT12) showed significant weight reduction compared to sedentary controls, indicating that endurance exercise may counteracts circadian disruption-induced weight gain independent of timing. In WT mice, evening exercise (ZT12) led to enhanced lipid regulation and better glucose tolerance. These time-dependent benefits were absent in Clock^Δ19 mutants, demonstrating that the full metabolic advantages of exercise require a functional circadian clock. Notably, endurance training also partially restored serum HDL-C levels in Clock^Δ19 mice, suggesting compensatory metabolic responses. Conclusions: Aligning endurance exercise with the body's internal clock provides greater metabolic benefits than untimed exercise. The circadian clock is essential for time-dependent improvements in glucose and lipid metabolism, although some beneficial effects occur independently of a functional clock.