Exercising in the morning rather than in the evening may be more beneficial to health, according to a study led by researchers at the University of California. The authors say their study supports previous research suggesting that the circadian rhythm is important in dictating how the body responds to physical activity.
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"Circadian rhythms dominate everything we do. Previous studies from our lab have suggested that at least 50% of our metabolism is circadian, and 50% 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," says senior author Paolo Sassone-Corsi.
Whilst it is widely accepted that eating at specific times of the day can affect the body’s metabolism, the effects of exercising at different times of the day have remained unknown.
Exercise stimulates metabolism, leading to the improvement of metabolic health. While the metabolic benefits from exercise have been extensively uncovered, the question of when it is appropriate to exercise has remained virtually unexplored.
For the study, the team placed mice on treadmills and assessed the impact that exercise had on skeletal muscle metabolism at different times of day.
Since mice are nocturnal animals, the team needed to translate mouse timing to human timing by comparing the animals’ active phase and resting phase rather than using a clock as a measure of the time of day.
Morning exercise improves oxygen circulation and burns more calories
As reported in the journal Cell Metabolism, the study indicated that exercising around mid-morning results in more oxygen being delivered to cells and yields a more rejuvenating effect on the body than exercising in the evening.
Using high-throughput transcriptomics and metabolomics to assess the animals’ muscle tissue, the researchers uncovered distinct changes in metabolism when the mice exercised during the early rest phase (equivalent to evening in humans), compared with when they exercised during the early active phase (morning).
Compared with evening exercise, morning exercise burned up more carbohydrates and ketone bodies and broke down more fats and amino acids. A protein called hypoxia-inducible factor 1-alpha (HIF-1α) played an important role, with exercise activating the protein in different ways depending on what “time of day” it was. This protein is a transcription factor that activates certain genes based on the level of oxygen in tissues.
Sassone-Corsi says that although it makes sense that HIF-1α would be important here, the fact that it fluctuates based on the time of day is a new finding.
Comparing mice to humans was not straight forward
The team notes that although circadian rhythms have been conserved throughout evolution, translating the findings to humans is still not straightforward. Humans have more variation in their chronotypes than mice, for example.
The term chronotype refers to a person’s natural inclination to sleep at certain times of the day. Although urges to sleep generally peak between 2 am and 4 am, the precise time varies between individuals and depends on other factors such as quantity or quality of sleep in the preceding days.
Teenagers have a delayed body clock
Researchers have found that teenagers tend to have a delayed circadian rhythm, compared with adults and children. Studies have shown that teenagers’ cortisol and melatonin levels peak later than usual, which can cause difficulty sleeping and lead to sleep deprivation.
"You may be a morning person, or you may be a night person, and those things have to be taken into account," says Sassone-Corsi.
Our results clearly indicate that time-of-day is a critical factor to amplify the beneficial impact of exercise on both metabolic pathways within skeletal muscle and systemic energy homeostasis.”