— -- As the five members of the U.S. gymnastics team flipped, twirled and defied the laws of gravity to win their gold medal on Tuesday afternoon, they may have been given a boost by an unlikely source: their circadian rhythm.
Multiple studies have found that the circadian rhythm -- a kind of biological clock that governs mood, appetite and sleepiness -- can affect an athletic performance. As a result, at certain points of the day, an athlete will biologically be at their peak for competition.
Alex Diamond, assistant professor of orthopedic surgery and rehabilitation at Vanderbilt University Medical Center, said there is one time of day that is best for both pro and amateur athletes to break records.
"Several studies show best time for physical performance is late afternoon, early evening," he said. "Certain hormones come out at certain times of the day. If they are staggered or altered, you get different impulses."
These hormones and biological processes, "can affect things like muscle strength and flexibility and nerve and motor control," Diamond said.
One key hormone at play is human growth hormone, or HGH. While the synthetic version is often associated with doping, HGH is also a naturally occurring hormone in the body that tends to spike in the afternoon and early evening. The hormone generally peaks overall when people are sleeping.
"It's important for strength and overall athleticism," Diamond explained. Increased levels of HGH can affect everything from exercise capacity to bone density and muscle mass, providing an extra boost during an athletic competition.
While there is a peak time of day for performance, there is also a mirror time when athletes will be far from their best. A study published this March in the medical journal Chronobiology International said events scheduled late into the night in the Summer Olympics could "lead to injury and may compromise an athlete’s decision-making, attentional, physiological and other processes."
The study authors called on specialists to work with athletes and consider "light exposure, melatonin intake, sleep hygiene and scheduled naps," to help them prep for the late night schedule.
Diamond said athletes taking part in sports scheduled for early morning or late night events, like swimming and beach volleyball, should still have hope for breaking personal records.
He said if athletes have to compete at midnight, they can rearrange their sleep schedule and "get used to midnight being [their] mid-afternoon."
Diamond also pointed out that getting enough sleep is key for an athlete to reach peak performance. Even if your competition is at a perfect time of day for an athlete hormonally, if they stayed up all night, they're less likely to perform well.
"We know that sleep in general is important for both mental and physical health," Diamond said. It also affects "cognitive perspective and the ability to make quick decisions and be alert."
Dr. Robert Meislin, an orthopedic surgeon and sports medicine physician at NYU Langone Medical Center, said if athletes know their event is either late in the evening or early in the morning, they can factor that into their training and schedule.
"The preparation will likely include the time of day they will be competing," Meislin said. "We have different levels of endorphins and catecholmaines [hormones from the adrenal glands] that peak and trough during the day."
In big international events like the Olympics, competitors can face an added burden by traveling long distances and grappling with jet lag before a competition. Meislin said it helps for athletes to acclimate to a time zone before competing and that athletes should "arrive at least one to two weeks beforehand to get sense of climate and timing."
While most of us are not facing off with Michael Phelps or Simone Biles, keeping your circadian rhythm in mind can help you reach a personal best in any athletic activity.