Is it better to exercise in the morning or afternoon?
That’s a bit of a trick question, because exercise at any time of day is better than no exercise.
But a major new review in the journal Trends in Endocrinology and Metabolism found that the timing of exercise can, in some instances, meaningfully influence how that exercise affects us.
In the review, researchers analyzed the results of multiple past studies of morning versus later-day exercise on the metabolisms of healthy people and those with Type 2 diabetes, a common condition affecting as many as 38 million American adults in which the body becomes resistant to insulin, resulting in consistently high blood sugar levels.
It found that many of our cells, hormones and genes respond differently, depending on when we work out.
The effects — and implications — proved to be especially striking for people with Type 2 diabetes, according to the analysis. For them, exercise later in the day tended to result in substantial and lasting improvements to blood sugar control.
The same exercise first thing in the morning, on the other hand, often led to increased blood sugar and impaired insulin responses.
“There’s a growing body of evidence suggesting that exercise later in the day may offer additional health benefits,” said Trine Moholdt, an exercise scientist at the Norwegian University of Science and Technology in Trondheim, Norway, who has studied exercise timing. She was not involved with the new review.
But any added benefits probably depend, the review also found, on who you are and how you work out.
Exercise affects circadian rhythms
One of exercise’s most-potent but least-appreciated powers is as a “zeitgeber” or “time giver” for our circadian rhythms, meaning it has the ability to influence those rhythms.
Normally, our bodies follow a 24-hour cycle that helps determine when we wake, sleep, eat and generally function. This cycle is orchestrated by molecular “clocks” in virtually every cell and organ in our body that coordinate with a central body clock in our brain.
This central clock is set and adjusted by “zeitgebers,” principally light and darkness, but also meal timing and exercise. In response to these cues, our internal clocks initiate intricate physiological processes that jump-start other bodily reactions, including the release of hormones, that then control many physiological operations.
As a result, our heart rates, blood pressure, insulin secretion, hunger, body temperature, energy, sleepiness, strength, cell division and many other systems crest and ebb in predictable patterns throughout the day and night.
But in people with Type 2 diabetes, these cycles often become disrupted, even upended. They can develop “circadian misalignment,” according to the review, meaning their various cellular clocks no longer communicate as well with each other, and some biological operations don’t occur when they normally should, if they occur at all.
For someone with circadian misalignment, exercise timing matters.
Morning exercise can raise blood sugar
When middle-age men with Type 2 diabetes began a program of relatively intense exercise either in the morning or afternoon in one of the studies analyzed in the review, the results were telling.
Their workouts had been identical at both times of day. But when researchers tested the men’s blood sugar levels afterward, they noted the afternoon workouts led to lower, healthier levels that lingered for up to 24 hours.
The same workouts completed in the morning left the men with higher blood sugar and worse insulin sensitivity that likewise lasted for hours.
Other experiments produced comparable results. “Most studies report higher postexercise glucose levels in individuals with type 2 diabetes … in the morning compared with the afternoon,” the authors of the new review wrote.
The role of the ‘dawn phenomenon’
Why should morning exercise seemingly exacerbate blood sugar problems in people with Type 2 diabetes?
Probably, said Harriet Wallberg-Henriksson, a professor of physiology at the Karolinska Institute in Stockholm and senior author of the new review, because of what’s known as the “dawn phenomenon,” which causes blood sugar to be especially high first thing in the morning.
In almost all of us, with or without normal blood sugar control, levels of cortisol, a stress-related hormone, spike first thing in the morning, she said. That jolt of cortisol helps us wake. It also prompts the liver to release stored sugar. In people with healthy metabolisms, the pancreas then pumps out insulin to nudge that blood sugar into our muscles, where it supplies fuel as we start to get up and move.
People with Type 2 diabetes produce less insulin and also are resistant to its effects, however, so their blood sugar levels typically rise and stay elevated first thing in the morning.
Working out early in the day, especially vigorously, such as with a run or intense bike ride, then usually raises cortisol levels even more and amplifies muscles’ need for fuel, leading to the release of more blood sugar.
That’s not a problem for people with healthy blood sugar control, whose muscles will burn through most of that sugar.
But people with Type 2 diabetes, who often are at their most insulin resistant first thing in the morning, Wallberg-Henriksson said, don’t move that extra blood sugar into their muscles well. Instead, it remains and continues rising in the bloodstream.
These conditions together “make morning exercise less favorable for promoting glucose metabolism in people with Type 2 diabetes,” she said, than the same workout later in the day.
Any exercise is better than none
But even morning exercise is still good for people with Type 2 diabetes, said Juleen Zierath, a physiologist at the Karolinska Institute and co-author of the review. “The most important message remains that exercise at any time is better than no exercise at all,” she said.
If you have Type 2 diabetes and prefer morning workouts, you might want to keep the intensity relatively low, Wallberg-Henriksson said. “Lighter physical activity, such as brisk walking, does not seem to confer a similar time-of-day effect” as more vigorous exertion.
Many questions are still open about the effects of exercise timing, though. Most of the studies included in the review were small, short-term and involved adult men. Will the metabolisms of women and older people respond in the same ways to early or late exercise? It’s not yet clear. This review also focused on blood sugar. But other issues, such as cardiovascular disease, sleepand longevity, could be affected by when we exercise.
Still, if you have Type 2 diabetes “and can choose, afternoon or evening exercise appears to be more beneficial,” Wallberg-Henriksson said.
But for all of us, “exercise benefits nearly every organ system in the body,” Moholdt said. “While timing may influence certain outcomes for certain groups, for most people, the best time to exercise is simply when they can fit it into their day.”
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