Morning Endurance Training Enhances Adaptation in Mice

Study Background and Research Question

Exercise performance and its physiological benefits are known to fluctuate with circadian rhythms, both in humans and animal models. Previous research has established that endurance capacity peaks in the late active phase, yet whether the timing of regular endurance training can modulate long-term performance adaptations remains unresolved. Given that glycogen storage, metabolic gene expression, and mitochondrial function all display circadian variation, the timing of exercise interventions could be a critical, yet underappreciated, variable in experimental and translational metabolic studies. Hesketh et al. (2026) specifically addressed whether morning (early active phase) versus afternoon (late active phase) endurance training differentially affects adaptation in mice, focusing on performance outcomes and underlying metabolic responses.

Key Innovation from the Reference Study

The fundamental innovation in the Hesketh et al. study is the direct comparison of long-term endurance training at distinct circadian times in a controlled murine model. Unlike previous short-term or cross-sectional investigations, this research employs a six-week training protocol—much longer than prior two-week studies—allowing for the observation of true adaptive processes rather than acute or transient effects. By integrating behavioral, biochemical, and molecular endpoints—including muscle and liver glycogen quantification, mitochondrial protein expression, and contractile phenotype—the authors provide a comprehensive analysis of how training time modulates adaptation efficiency and metabolic remodeling.

Methods and Experimental Design Insights

Female C57BL/6J mice were randomly assigned to treadmill endurance training during either the early active phase (zeitgeber time 13, ZT13, corresponding to morning) or late active phase (ZT22, afternoon), five days per week for six weeks. Training intensity was standardized at 70% of each animal's initial maximal capacity, with performance assessments conducted at baseline, week three, and week six. Secondary measures included cage activity, body composition (fat and lean mass), food intake, blood glucose and lactate, and tissue glycogen content. Molecular analyses of skeletal muscle samples included citrate synthase activity, COXIV protein levels, and myosin heavy chain (MyHC) isoform distribution, providing mechanistic insight into mitochondrial and contractile remodeling.

Protocol Parameters

• Training frequency: 5 days per week for 6 weeks (morning or afternoon sessions).
• Training intensity: 70% of each mouse's maximal running capacity, individualized for baseline fitness.
• Performance assessment: Conducted at baseline, week 3, and week 6 using treadmill endurance tests.
• Body composition: Fat and lean mass measured using standard non-invasive techniques.
• Glycogen quantification: Liver and skeletal muscle samples collected post-training; measurements performed with established colorimetric or enzymatic assays.
• Mitochondrial and contractile profiling: Citrate synthase activity, COXIV protein quantification, and MyHC isoform analyses via immunoblotting or enzyme assays.

Core Findings and Why They Matter

At baseline, mice tested in the afternoon (ZT22) exhibited higher endurance performance compared to those tested in the morning (ZT13), consistent with established circadian patterns. However, after six weeks of training, the morning-trained (ZT13) mice displayed a markedly greater rate of improvement—endurance performance increased by 132% in this group versus only 45% in the afternoon-trained cohort, despite the former achieving lower absolute training volumes (Hesketh et al., 2026). By week six, both groups reached comparable absolute performance levels, but the morning-trained mice attained these gains more efficiently.

Both training regimens led to significant reductions in fat mass (31% and 32% decreases for ZT13 and ZT22, respectively), without significant differences in lean mass, food intake, or muscle and liver glycogen content between the groups. Notably, morning training was associated with increased COXIV protein expression, enhanced citrate synthase activity, and shifts in skeletal muscle MyHC isoform composition, suggesting superior mitochondrial and contractile adaptation in response to early-day exercise stimuli. These findings collectively indicate that the timing of endurance training is a biologically relevant modulator of adaptive efficiency, with morning sessions eliciting more robust molecular and functional remodeling in skeletal muscle.

Comparison with Existing Internal Articles

The present findings align with and extend insights from internal commentaries, such as "Morning Training Enhances Endurance Adaptation in Mice" and "Morning Endurance Training Enhances Muscle Adaptation in Mice", both of which discuss the pronounced impact of exercise timing on physiological adaptation. These articles underscore that early active phase training can yield superior performance gains and muscle remodeling, corroborating the present study's conclusions. Additionally, internal resources such as "Glycogen Colorimetric Assay Kit II: Advancing Glycogen Quantification" emphasize the importance of reliable, interference-resistant glycogen measurement tools for dissecting metabolic responses to circadian and exercise interventions. The present study's rigorous glycogen quantification parallels these methodological recommendations, ensuring robust interpretation of metabolic phenotype data.

Limitations and Transferability

While the study provides compelling evidence for the importance of exercise timing in murine models, several limitations temper direct translation to human physiology. The experiments were conducted in female mice only, and the circadian architecture in nocturnal rodents differs from that of humans, potentially influencing the directionality of optimal training windows. Moreover, training was performed under tightly controlled laboratory conditions, which may not fully recapitulate free-living environments. The absence of significant differences in muscle and liver glycogen content between groups at study end raises questions about the sensitivity and timing of tissue sampling relative to exercise, as well as the possible need for more sensitive glycogen hydrolysis assays or temporal profiling. Nonetheless, the demonstration of molecular and functional adaptation differences supports the broader concept that circadian context is a critical determinant of exercise response variability.

Research Support Resources

For researchers seeking to replicate or extend these findings, precise quantification of tissue glycogen is essential to link metabolic adaptation to exercise timing. The Glycogen Colorimetric Assay Kit II (SKU K2144) from APExBIO provides a sensitive, high-throughput solution for quantifying glycogen in biological samples, with robust performance even in the presence of reducing substances that may interfere with oxidase-based assays. This kit is suitable for workflows involving time-of-day or endurance training protocols, and its enzymatic hydrolysis and colorimetric detection can support studies aiming to dissect subtle metabolic phenotypes, including those relevant to glycogen storage disease research or circadian metabolism. For optimal results, the kit should be stored at -20°C as per manufacturer guidance.