1. Exercise can help your circadian rhythms stay in sync

Several activities, including traveling, staying up too late, and eating at unusual times, can disrupt your circadian rhythms and lead to what scientists call "circadian misalignment." This misalignment has been associated with several health conditions, such as cancer, diabetes, cardiovascular disease, and psychiatric illness.

"Under normal conditions, our clocks are aligned — they're in the same time zone," said Karyn Esser, chair of physiology and aging at the University of Florida's College of Medicine. "Problems arise when our clocks are misaligned, so the brain clock thinks it's here, and the liver and the pancreas think it's something else."

While light is the most effective way to align your circadian rhythm with the outside world, exercise can also help synchronize your body with its sleep-wake cycle. For example, some recent clinical trials have found that exercising in the morning or early afternoon can push your circadian rhythms to an earlier schedule.

In comparison, exercising in the evening seems to delay your biological clock. However, this finding may not be true for people who like to stay up late at night.

2. Your muscles have their own circadian rhythms, which can affect performance

According to Katja Lamia, an associate professor of molecular and cell biology at Scripps Research, "[e]very cell in our body has a circadian clock," which "means so many aspects of our physiology are different — at different times of day."

Muscle clocks are key to letting a cell know when to rest and repair, store energy, and prepare for activity. These clocks are also directly linked to our metabolism and how we use energy.

The efficiency of the mitochondria, which powers cells, also fluctuates throughout the day, with research finding that energy peaks in the late afternoon. These fluctuations in the muscles may also explain a longstanding find that most people tend to have better physical performance later in the day.

"The evidence is very suggestive that circadian clocks in the muscle and probably in other organs are important for driving these differences in exercise capabilities," Lamia said. "It just hasn't been proven yet."

Although the mice study has yet to be published and has limitations, the idea that consistency can have a positive impact on performance has also been found in human data, Stone writes.

"There's pretty good evidence that if you exercise regularly at the same time of day, then you tend to improve your performance the most at that time," Lamia said. 

4. When you work out can impact your health

Currently, scientists are trying to figure out what exercise time would be the most beneficial for people with medical conditions, including metabolic syndrome or diabetes. Research shows that circadian disruption can negatively impact both blood sugar and insulin levels, and exercising at certain times of the day could help people have better control of their metabolism.

"We're trying to use exercise to basically reset the rhythms" of the muscles’ clocks, which would then "reset their metabolism," said Juleen Zierath, a professor of physiology at the Karolinska Institute in Sweden.

So far, research on the topic has had mixed results, with some studies finding that exercising in the afternoon can have health benefits for people with metabolic impairments and others finding that exercising in the morning may be better.

"In general, we're still at a pretty early stage of trying to understand this," Lamia said. However, she also noted that "the best time to exercise is whenever you're actually going to do it."