Body clocks matter for heart health

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As modern lifestyles increasingly challenge the body’s natural circadian rhythm, the American Heart Association is spotlighting the potential health consequences of regular disruptions to our body’s internal clock with a new scientific statement published today in its journal Circulation.

Heart and lungs illustration. Human Chest Cavity illustration: Right lung, left lung, heart. Copyright American Heart Association

The statement, “Role of Circadian Health in Cardiometabolic Health and Disease Risk,” outlines how disruptions to circadian rhythm may increase the risk of cardiovascular disease and health risk factors including obesity, Type 2 diabetes and high blood pressure. Additionally, circadian disruption may be particularly relevant to cardiovascular–kidney–metabolic health, which is associated with cardiovascular disease incidence and mortality risk.

“Circadian rhythms are produced by the body’s natural 24-hour internal clock that regulates a multitude of biological processes like sleep, wakefulness, hormone release, digestion and body temperature. Regular interruptions to the body clock are much more than simple inconveniences like staying up too late or waking up too early. These disruptions may trigger adverse health effects in several different ways,” said Kristen Knutson, Ph.D., FAHA, volunteer chair of the statement writing group, volunteer chair-elect of the Association’s Sleep Science Committee and an associate professor of neurology at Northwestern University’s Feinberg School of Medicine in Chicago. “Aligning our daily behaviors, when we sleep, eat and move, with our internal clock is important to support optimal cardiometabolic health.”

Circadian rhythms are approximately 24-hour cycles in physiological processes that regulate key functions such as heart rate, blood pressure, metabolism and hormone secretion such as cortisol, important for stress response and metabolism. The statement defines circadian health as the optimal function, rhythmical character and alignment of the circadian system with the light–dark cycle. The circadian system is synchronized primarily though light detected by the retina in the eye and transmitted to the suprachiasmatic nucleus (special neurons in the brain’s hypothalamus region). Inside these neurons and throughout the body are special genes and proteins (like CLOCK, BMAL1, PER and CRY) that help generate rhythmic activity. They turn on and off in a regular cycle, creating the rhythm of the body’s internal clock.

The timing of when an individual sleeps, eats and engages in physical activity are among the behaviors that can influence this alignment. Circadian disruption can occur if a person’s internal body clock does not align with their behavior. For example, if an evening person must wake earlier than their body clock expects, perhaps on an overseas trip or for an early morning job, they will be awake during their biologic night, which could disrupt circadian rhythms.

Knutson, who specializes in sleep and circadian research specifically focused on associations with cardiometabolic and cognitive health, noted, “although circadian rhythms play an important role in keeping us healthy, they are often overlooked in everyday medical care. It’s important to realize what these disruptions can do to our body and how to lessen any impact.”

Key Takeaways:
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Sleep timing regularity is as important as sleep duration: Irregular sleep schedules, even with adequate sleep length, can disrupt circadian rhythms. “Social jet lag” or variations in sleep schedule on work or school days vs. free days has been linked to a risk of obesity/overweight. The timing and regularity of sleep also play a role in diabetes, with social jet lag and greater day-to-day variability in sleep duration and timing emerging as risk factors for glycemic dysregulation and Type 2 diabetes. Maintaining consistent sleep and wake times helps synchronize the body’s internal clock and supports metabolic health. Sleep is one of the key measures for improving and maintaining heart and brain health in the [Association’s Life’s Essential 8](Life’s Essential 8)TM.
Light exposure timing is a therapeutic tool: Light is the primary cue or synchronizer for the central circadian clock in the brain. Morning exposure to natural light helps reinforce healthy rhythms, while exposure to artificial light at night—especially blue light from screens—can suppress melatonin and delay sleep onset. Even low levels of nighttime light have been associated with increased risk of cardiovascular disease.
Meal timing affects metabolic health beyond caloric content: Eating late at night or having irregular mealtimes can misalign the circadian clocks found in organs like the liver and pancreas, contributing to blood sugar spikes or dips and weight gain. Studies show that eating earlier in the day, such as having breakfast before 8:00 a.m., is associated with lower risk of Type 2 diabetes and better cardiometabolic outcomes.
Physical activity timing may enhance circadian health: Exercise acts as a secondary synchronizer. Morning or afternoon workouts can help advance circadian rhythms, while evening exercise may delay them. The timing of physical activity may also influence outcomes like blood pressure, glucose (blood sugar) control and sleep quality, though more research is needed to determine optimal timing for different individuals.
Circadian misalignment is amplified by shift work, light pollution and inconsistent sleep: Individuals working nontraditional schedules, such as shift workers, may face increased exposure to light pollution, irregular mealtimes and inconsistent sleep patterns compared to those who have more traditional day/night schedules. Misalignment from rotating and night shift work is an established cardiovascular disease risk factor.
Individual internal clock timing should guide timing of interventions: Chronotype, a person’s internal timing for sleep and activity, commonly described as “morning person” or “night owl,” affects how individuals respond to light, meals and exercise. Tailoring interventions to a person’s natural timing may improve effectiveness and support circadian alignment. However, assessing true circadian rhythms is challenging.

Currently, researchers can only measure circadian rhythms accurately in small, highly controlled lab settings. In everyday health care, doctors often rely on simple clues, such as what time a person says they go to bed, which aren’t always precise. New technologies may soon make it easier. Tools like wearable devices, advanced lab tests and artificial intelligence could help track patterns in things like skin temperature and heart rate over 24 hours, giving a clearer picture of an individual’s natural body clock rhythm.

“Everyone has an internal clock, and it’s time we start listening to it,” Knutson said. “Simple changes, like going to bed and waking up at the same time every day, eating meals earlier and getting morning sunlight, may make a meaningful difference in your heart and metabolic health. We still need more research to establish causality and understand how and why circadian rhythms affect health. Ideally, such research may help scientists figure out the best times for healthy habits like sleeping, exercising and eating, and create personalized strategies that match people’s natural body clocks.”

Citation
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The study Role of Circadian Health in Cardiometabolic Health and Disease Risk: A Scientific Statement From the American Heart Association was published in AHS’ journal Circulation. Authors: Kristen L. Knutson, PhD, FAHA, Chair, Debra D. Dixon, MD, MS, Michael A. Grandner, PhD, MTR, FAHA, Chandra L. Jackson, PhD, MS, Christopher E. Kline, PhD, FAHA, Lisa Maher, DNP, Nour Makarem, PhD, FAHA, Tami A. Martino, PhD, Marie-Pierre St-Onge, PhD, FAHA, and Dayna A. Johnson, PhD, MPH, FAHA, Vice Chair on behalf of the American Heart Association Council on Lifestyle and Cardiometabolic Health; Council on Cardiovascular and Stroke Nursing; Council on Clinical Cardiology; and Council on Lifelong Congenital Heart Disease and Heart Health in the Young

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