When it comes to daily exercise routines, most of us decide what time of day we can break a sweat based on real-world logistics (e. ., a 9-to-5 work schedule) more than our internal circadian rhythms. During the week, most gym-goers will exercise and take a post-workout shower before commuting to work or they'll hit the gym after work and wash-up before heading home for dinner. Some people manage to squeeze in a workout during their lunch break. When it comes to sticking with a daily exercise routine, the most important thing is to find a time of day that works best for you and to make daily exercise a regular habit.

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That said, a new study (Sato et al., 2019) on mice found that time of day and circadian rhythms may influence how aerobic exercise affects metabolism. The research suggests that morning exercise significantly increases the ability of muscle cells to metabolize sugar and fat, while evening exercise appears to boost overall metabolism for a longer duration of time.

This paper, "Time of Exercise Specifies the Impact on Muscle Metabolic Pathways and Systemic Energy Homeostasis," was recently published in the journal Cell Metabolism. The research was conducted by scientists from the Treebak Group at the University of Copenhagen in with colleagues at the University of California, Irvine.

For this mouse study, the researchers used a state-of-the-art transcriptomic and metabolomic approach to identify distinct metabolic oscillations corresponding to morning and evening exercise as related to circadian rhythms. Their findings show that metabolic responses appear to be controlled by a central circadian clock mechanism that uses a protein called "HIF1α."

As the authors explain, "At the molecular level, HIF1α, a central regulator of glycolysis during hypoxia, is selectively activated in a time-dependent manner upon exercise, resulting in carbohydrate exhaustion, usage of alternative energy sources, and adaptation of systemic energy expenditure."

When Should You Exercise for the Best Metabolic Outcomes?

In a videotaped interview (watch here), one of the study's coauthors Jonas Thue Treebak of the Novo Nordisk Foundation Center for Basic Metabolic Research, makes it clear that their research does not conclude if morning or evening exercise is "better" for metabolism; both morning and evening exercise have metabolic benefits.

"We cannot say for certain which is best, exercise in the morning or exercise in the evening. At this point, we can only conclude that the effects of the two appear to differ, and we certainly have to do more work to determine the potential mechanisms for the beneficial effects of exercise training performed at these two time-points," Treebak said in a statement. "We are eager to extend these studies to humans to identify if timed exercise can be used as a treatment strategy for people with metabolic diseases."

While we wait for more research to help us better understand how working out in the morning or evening affects metabolic processing, my advice is to exercise during any time of day that fits your schedule.

There are countless other benefits associated with exercising in the morning, evening, or other times of day that go far beyond metabolism. For example, some people like to kickstart their morning with some exercise because it puts them in a good mood for the rest of the day. Also, a recent study (Wheeler et al., 2019) found that 30 minutes of morning exercise may improve decision-making during the day.

Some people use a vigorous evening workout to let off steam after a day at the office; others like to engage in easy-to-moderate physical activity such as an evening stroll (i.e., la passeggiata) as a winding-down ritual before bed. Again, please exercise whenever it fits your lifestyle and works best for you.

Author's note: As a science reporter and blogger, I find that morning exercise helps me connect the dots of seemingly unrelated studies, process empirical evidence I've read in the past 24 hours, and figure out how new research fits into the bigger picture. As a real-time example, I first read about the new Sato et al. (2019) study in the predawn hours this morning and then went for a long jog at sunrise. While I was running, I created an outline for the post you're reading now in my head and visualized ending with two other morning/evening exercise studies and the diagram below.

Source: ETH Zurich / Jan Stutz

There is one caveat about evening exercise: As you can see in the diagram above, a meta-analysis (Stutz et al., 2018) found that easy-to-moderate exercise before bedtime promotes deep sleep. However, if you are considering a high-intensity evening workout as a way to possibly boost your metabolism for a longer duration of time, take note that vigorous exercise two hours or less before you hit the sack could cause .

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