Scientists are increasingly finding that the circadian rhythm — i.e. our internal body clock — within each of our tissues plays an important role in how efficiently they perform at different times of the day.
For example, research suggests athletes tend to perform best in the late afternoon and early evening.
When Dutch sports scientists examined the performances of swimmers at the Athens, Beijing, London and Rio Olympics, they found they achieved their fastest times at around 5pm. It’s thought these principles hold true for all of us.
"Exercise timing might play a role in fine-tuning some of the outcomes of exercise," says Juleen Zierath, a professor of integrative physiology at Karolinska Institute in Sweden, and a leading expert on the connection between body clock and biology.
"A lot of our physiology — from body temperature to heart rate and genes that control the metabolism of fat — operates on a 24-hour cycle."
Scientists are beginning to learn how all tissues and organs have their own unique schedules in accordance with fluctuations of different hormones.
For while our muscles are at their most powerful and flexible later in the day, new research demonstrates that we can burn fat more efficiently when we exercise in the morning.
Professor Zierath says there are thousands of genes which produce enzymes that help convert fat stored in cells into the energy we use when we exercise. And it seems these genes are at their most active in the morning.
So while you may not be at your strongest and quickest then, it is the best period for losing weight.
In a recent study, published in the journal Proceedings of the National Academy of Sciences, Professor Zierath and colleagues investigated this for the first time, using mice.
They found that when the mice exercised on a treadmill in the first three hours after waking, their body fat showed much higher levels of the enzymes used to metabolise fat.
When they did the same amount of exercise much later in the day, there were far lower levels of these enzymes.
"The implication is that if you’re a regular morning exerciser, your body is likely to be more sensitive to breaking down fat and using it as energy," says Professor Zierath.
"And what that could mean is that one might potentially be able to lose a bit more weight. Exercise also helps with weight maintenance, so morning exercise may prevent weight gain."
Scientists suspect the body’s preference for burning fat after morning exercise is linked to the natural pattern of hormones called glucocorticoids, which control how the body maintains and accumulates fat.
Glucocorticoids are found throughout the body, most notably in fat tissue. Their natural pattern is to peak around 8am, which gets us moving and switches on our appetites, before reaching their lowest level around 3am when we are fast asleep.
Exercising in the morning fits nicely with this daily cycle, as glucocorticoid levels are already raised, and so the body does not generate any additional fat cells.
However, if we exercise in the evening, this causes a surge of glucocorticoids, triggering the body to produce new fat cells to replace some of the ones you have just burnt, minimising the benefit to your waistline.
This may help explain why shift workers are more prone to weight gain: their nocturnal patterns of being stressed and active cause big surges in glucocorticoids at times when the body would normally be asleep, leading to more fat cells being created.
And as a result, shift workers are more likely to gain weight compared with people who have normal circadian patterns, even if they eat a similar number of calories a day.
This is the consequence of evolution, explains Dr Mary Teruel, an associate professor of biochemistry at Weill Cornell Medicine in New York, and a world-leading expert on glucocorticoids. She points to how our bodies have developed around a pattern of being awake during the day and asleep at night.
"Fat is strongly influenced by hormones," says Dr Teruel. "If the levels of glucocorticoids are disrupted, that has a huge effect on fat.
"If you exercise later, these hormones start being secreted to a much greater extent in the evening, which isn’t good," she explains.
"Your body wants to rest at this time, and if you pump it up it’s equivalent to staying up really late and not sleeping."
Instead, Dr Teruel suggests that a morning workout, no matter the length or type of exercise, will yield greater benefits for losing weight.
However, working out post-lunch can have different benefits. Professor Zierath says your blood sugar will be more stable if you exercise in the afternoon — meaning it’s the best time for people who either have type 2 diabetes or are at risk of developing it.
This is important because if your blood sugar fluctuates too much, the excess sugar starts to be stored as fat.
Professor Zierath says: "We’ve studied people with type 2 diabetes and found that when they did high-intensity interval training in the afternoon, this led to the most optimal improvement in their blood sugar control compared with morning or evening exercise."
She also points to a study of more than 92,000 people, published in the journal Nature Communications in February, which found those who exercised between 11am and 5pm were less likely to die of heart disease, compared with those who exercised earlier in the day or later at night, possibly due to the link between blood sugar stability and heart health.
But she emphasises that while morning or daytime exercise might have the most optimal outcomes, exercise at any time is better than none at all.
The Nature Communications study demonstrated that people who frequently engaged in moderate-to-vigorous physical activity lived longer than those who did barely anything.
"You have to listen to your body because there are many people who say they prefer being evening runners, while others prefer being morning runners," says Professor Zierath.
She feels that while the evidence might suggest that a morning workout can offer more short-term benefits for losing weight, it is better to have a routine which enables you to be a consistent exerciser.
"That’s really important," she says. "But you might be able to boost the beneficial effects of exercise by doing it at certain times."
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