Two new studies have suggested that the circadian clock could play a more important role in how the body responds to physical exertion than previously thought.

The reports, published in the April issue of the journal Cell Metabolism, looked at the association between time of day and exercise performance in both mice and humans.

The studies focused on different components of exercise, which means the findings complement each other – but also resulted in somewhat conflicting results.

One group of researchers, led by Gad Asher at the Weizmann Institute of Science, placed mice in treadmills at different times of day within their active phase.

They examined the exercise capacity of mice upon different exercise intensities and regimens and found that overall exercise performance is substantially better in the "mouse evening" (toward the end of their active time) compared to the morning hours.

The researchers also studied 12 humans and found similar effects. Overall, the people in the study had lower oxygen consumption while exercising in the evening compared with the morning; this translated to better exercise efficiency.

"It's quite well known that almost every aspect of our physiology and metabolism is dictated by the circadian clock," said Gad Asher, senior author of the study.

"This is true not only in humans but in every organism that is sensitive to light."

Another group of researchers, led by Paolo Sassone-Corsi of the Center for Epigenetics and Metabolism at the University of California, Irvine, also studied mice on treadmills – but had a different approach.

Through the use of high-throughput transcriptomics and metabolomics, the team looked at the changes in the mice's muscle tissue that occur in response to exercise. This allowed researchers to look at processes such as glycolysis – which contributes to sugar metabolism and energy production – and lipid oxidation (fat burning).

The study found that a protein called hypoxia-inducible factor 1-alpha (HIF-1) – a transcription factor which stimulates genes based on oxygen levels in tissue – is activated by exercise in different ways depending on the time of day.

Based on the work from the UC Irvine team, exercise seemed to have the most beneficial impact on the metabolism at the beginning of the active phase – equivalent to late morning in humans – compared with the resting phase (evening).

Sassone-Corsi said: "Circadian rhythms dominate everything we do.

"Previous studies from our lab have suggested that at least 50 per cent of our metabolism is circadian, and 50 per cent of the metabolites in our body oscillate based on the circadian cycle. It makes sense that exercise would be one of the things that's impacted."

To read the full research paper, click here for Cell Metabolism.