In the study, which was published in Cancer Epidemiology in September, the researchers analyzed data from 2,795 people divided into two control groups and two cancer groups.
The results indicated that the protective effect against breast and prostate cancer was most significant in the people who exercised in the morning (between the hours of 8 a.m. and 10 a.m.). The results held even when the researchers adjusted for other factors, such as participants’ sleep routines and meal timing.
“Researchers have classified circadian disruption as a carcinogen, not unlike tobacco or asbestos,” Sean Marchese, MS, RN, a registered nurse and oncology writer at The Mesothelioma Center, tells Verywell. Marchese was not involved with the study.
“Circadian regulatory functions are critical for many aspects of homeostasis, the body’s method of regulation,” Marchese says. “A disruptive circadian rhythm can impair cell cycles, DNA repair, and metabolism. Errors in any of these processes can lead to cancer, especially if disruptions continue to occur over a long period.”
What Is Your Circadian Rhythm?
Your circadian rhythm, also known as your sleep/wake cycle, is a 24-hour internal clock that is controlled by your brain. It tells you when it’s time to go to sleep and when it’s time to wake up. The rhythm is regulated by various hormones, two of the most notable being cortisol and melatonin.
Morning chronotypes wake up early and go to bed early, while evening chronotypes wake up later and go to bed later. Evening chronotypes usually have a sleep pattern that’s about two to three hours behind morning chronotypes. As you might expect, intermediate chronotypes fall somewhere in the middle.
Sleep patterns are also associated with different hormonal fluctuations. Morning chronotypes have an earlier onset of melatonin production; their levels rise earlier in the evening and prompt their earlier bedtimes. For the intermediate and evening chronotypes, melatonin rises later and leads to later bedtimes.
The differences matter in the context of the recent study because the researchers found that the protective effects of early morning exercise were more significant in intermediate and evening chronotypes than in morning chronotypes.
The researchers speculated that midday to afternoon exercise might delay a person’s melatonin production even more if they were an intermediate or evening chronotype. Therefore, earlier exercise could lead to more melatonin, a balanced circadian rhythm, and reduced cancer risk.
Keeping Your Circadian Rhythm Balanced
When your circadian rhythm is thrown off, it can lead to difficulty sleeping through the night, insomnia, fatigue, and energy crashes during the day. Evidence has also connected a disrupted circadian rhythm to imbalanced blood sugar, negative changes in metabolism, weight gain, depression, dementia, and cancer.
Marchese adds that getting enough quality sleep and trying to keep a regular sleep schedule are also critical factors—ones that you might be able to improve by taking a closer look at your sleep habits.
“Try not to be on your phone or other screen-based devices before bed or if you wake up in the middle of the night,” Marchese says. “That light at night is what triggers the melatonin suppression and disruption to your circadian rhythm.”
What’s More Important: Sleep or Exercise?
Ideally, you would get adequate amounts of both, but sometimes you have to choose one over the other.
As far as your circadian rhythm is concerned, the answer is likely to make sure you’re getting enough sleep. “Sufficient sleep is critical for circadian homeostasis,” Swathi Gujral, PhD, a postdoctoral fellow at the University of Pittsburgh’s Brain Aging & Cognitive Health Lab, tells Verywell. “The deleterious effects for overall health of circadian cycle disruptions due to poor sleep likely outweigh the health benefits of exercising specifically in the morning hours.”
Alpa Patel, PhD, the senior vice president of population science for the American Cancer Society, tells Verywell that rather than sacrificing good sleep and putting pressure on yourself to work out in the morning, getting regular exercise—whenever you can—is what’s most important.
Future Research
While Patel calls the new study intriguing, she adds that there is still more research to be done. While exercise is connected to a more balanced circadian rhythm and reduced risk of cancer, the time of day might be less important than getting regular exercise, no matter what the clock says.
Patel also points out that the results of the study could have been influenced by other factors known as residual confounding factors and their overall effect on your health. While the researchers tried to account for some of these factors (such as participants’ total calorie intake), there were others (such as participants’ overall diet quality) that were missing. Therefore, additional research is needed before any definitive claims could be made.
Gujral adds that you should be aware of and sensitive to how your environmental and social exposures in your daily life could be disrupting your circadian rhythm—especially if these exposures are chronic and/or persistent, such as in the case of long-term shift work.
