Peak workout performance depends on sleep, nutrition, and increasingly, what time of day we work out. As scientists do more research on our internal clocks, we’re learning that timing a workout to coincide with the body’s natural clock can help us get the most out of training.

A study released Thursday in Cell Metabolism points to the internal clock’s powerful effects on exercise. Lead study author Gad Asher, Ph.D., of Israel’s Weizmann Institute of Science, explains that his data from both human and mouse experiments shows exercising later in the day may increase workout efficiency in humans, making it easier to go harder. The mouse experiment gives us a sense of why afternoon workouts may be easier: Different molecular processes are happening in muscle cells during that time, priming those muscles to help you really go for it.

"In both cases, exercise capacity was higher in the evening hours compared to the morning hours."

“The human data is consistent with the mouse data, at least for the medium exercise intensity protocol,” he tells Inverse. “In both cases, exercise capacity was higher in the evening hours compared to the morning hours.”

What Happens When You Exercise Later in the Day?

In the human portion of Asher’s paper, he had participants perform an hour-long cycling test at either 8 a.m. or 6 p.m. These two times represent two different ends of the circadian clock’s “active phase,” which means that the thousands of genes that govern our internal rhythm are telling the body it’s time to be alert and ready to go.

Asher found that when humans exercise during the 6 p.m. time slot, they consumed significantly less oxygen and had lower heart rates for the same amount of work. In short, he found that their bodies performed more efficiently during that later time period. The human subjects also reported lower levels of perceived exertion for the same tasks later in their active cycles — suggesting that working out felt easier psychologically, too.

Humans found it easier to work out in the afternoon than in the morning. Cell Metabolism

His mouse study gives us an idea of why this may be the case. Mice who exercise during the later portions of their active phase were also able to run longer during treadmill tests, Asher notes, perhaps in part because they’re burning different fuel sources to produce that energy.

“Based on our mice work it appears that the efficiency of fatty acid oxidation and glycolisis upon exercise is higher in the evening hours compared to the morning, this might explain the better performance in the evening,” he says.

In other words, the body has an easier time converting both fats and carbohydrates into energy later in the day. Importantly, they note that there were no differences in food intake between those two times, suggesting that there really were different things going on on a cellular level that may explain this effect.

The Benefits of Timing Exercise With Your Internal Clock

As part of the experiment, Asher analyzed the muscle tissue of his mice. There, he found that mice who exercise later in the day tended to accumulate more of a metabolite called ZMP or 5-aminoimidazole-4-carboxamide ribonucleotide when they worked out. Essentially these high levels of ZMP trigger another reaction in the body: the activation of an enzyme called AMPK.

Working out during the later portion of the "active phase" may make it easier to go harder. For Asher's subjects, that corresponded to around 6 p.m. Pxhere

Asher calls AMPK the “master metabolic regulator” because it acts as the body’s energy sensor. If cells need more energy (like during exercise), it tells the body to start turning carbohydrates or fats into energy. AMPK is typically activated during a workout, but that activation seems to be amplified when the workout coincides with the body’s “active phase.”

“AMPK, a master metabolic regulator, accumulates upon exercise to higher levels in the evening compared to the morning,” he says. “Given that AMPK regulates fatty acid oxidation and glycolysis, we believe that the higher induction of ZMP in the evening contributes to the improved exercise performance.”

The fact that ZMP seems to accumulate more in the evening than in the morning points to the idea that it is, to some extent, reliant on the body’s circadian rhythm, explains Asher. The way to hack your cells to produce this effect during exercise is to perform exercise during the latter portion of the body’s “active phase” — which Asher’s study took to be around 6 p.m. However, that time isn’t set in stone, because each person’s active phase may be slightly different.

More recent work has shown that there are two “chronotypes” that govern our circadian rhythms. “Night owls” are people who feel more awake at night, while some “morning larks” feel more awake in the morning. Night owls and morning larks likely enter the “active phases” at different times of day, during which exercise may feel easier for them. Asher’s study hasn’t tested how exercise differs between chronotypes, but he does point out that it’s conceivable there will be different optimal workout times for morning larks and night owls:

“We do have evidence from the mouse work that the daily variance in exercise performance depends on the molecular circadian clocks, so it’s conceivable that indeed different chronotypes will show differences,” he says.

While we’re still waiting for specific data on morning larks and night owls, this paper is good news for anyone who finds it hard to hit a personal best in the morning. Just waiting a few hours might make all the difference.

Summary: Physical performance relies on the concerted action of myriad responses, many of which are under circadian clock control. Little is known, however, regarding the time-dependent effect on exercise performance at the molecular level. We found that both mice and humans exhibit daytime variance in exercise capacity between the early and late part of their active phase. The daytime variance in mice was dependent on exercise intensity and relied on the circadian clock proteins PER1/2. High-throughput gene expression and metabolic profiling of skeletal muscle revealed metabolic pathways that are differently activated upon exercise in a daytime-dependent manner. Remarkably, we discovered that ZMP, an endogenous AMPK activator, is induced by exercise in a time-dependent manner to regulate key steps in glycolytic and fatty acid oxidation path- ways and potentially enhance exercise capacity. Overall, we propose that time of day is a major modifier of exercise capacity and associated metabolic pathways.