Time of day that we sleep is critical to cardiovascular health but may also play part in effectiveness of exercise

We all know that exercise and rest are imperative for our mental, emotional and physical health. But what if when we exercise and sleep is more crucial to our health than we realize, especially when it comes to our heart health?

Associate Professor of Exercise Science Dr. Hayley MacDonald and her doctoral student Grant Malone are currently working on a study that aims to investigate the effects of exercise timing and chronotype, or a person’s preference for the timing of sleep and activity, on cardiovascular health. This study, part of Malone’s dissertation, is part of a larger initiative called the “Cardiovascular Health, Exercise, Circadian Rhythm & Sleep” Study or “CHECRS.”

“The more we study it, the more we see that individuals have very different responses to exercise. Not everyone responds the way you think they should,” MacDonald said. For example, some individuals are considered “super responders” (have a very favorable response to exercise) while others may see very little change if any at all, she added. “I’m one of many researchers trying to figure out what factors influence individual responses to exercise and how to optimize cardiovascular health outcomes.”

With a background in cardiovascular physiology, specifically among individuals with hypertension and heightened cardiometabolic risk, MacDonald has more recently focused her work on better understanding the relationships amongst cardiovascular health, exercise and chronotype (circadian rhythm and sleep). Chronotype is typically categorized as morning, evening or intermediate, where morning chronotypes naturally wake up and fall asleep earlier than evening chronotypes, exhibiting activity early in the day compared to evening chronotypes.

The importance of circadian rhythm is becoming increasingly apparent as there is a growing body of literature supporting that circadian disruptions, or mismatches between the body’s internal clock and external environment, are associated with physiological dysfunction, she said

A more specialized extension of MacDonald's work is examining this phenomenon among people who work high-stress professions, such as firefighters or police officers, where late nights and shift work are typically common.

We assume these individuals would fall under the evening chronotype, preferring to wake up later and go to sleep later, she said, but that’s not necessarily the case. For those who fall under the morning chronotype, shift work could be particularly detrimental because of the major misalignments with their preferred wake-sleep cycles.

“We know shift work throws the body’s internal clock in disarray … and that has devastating implications for cardiovascular and metabolic health, essentially, all the major systems involved in how the body is regulated.”

Studies suggest that those who align more as evening chronotypes exhibit poorer cardiovascular and metabolic health profiles. In fact, the American Heart Association added sleep to its Life’s Essential 8 checklist a few years ago, recommending that adults need seven to nine hours of sleep each night for optimal cardiovascular health.

“And while sleep duration is only one aspect of sleep health, a lack of sleep or disrupted sleep affects every system of the body,” MacDonald continued. “How sleep timing and sleep quality aligns with our internal circadian rhythm and our external environment is likely key to understanding when and how these misalignments occur.”

For example, much of our population maintains a standard 8- or 9-to-5 schedule, but that may not align with one’s chronotype. The terms “early bird” and “night owl” are often used to describe a person’s preference for waking and sleeping habits, but perhaps it is more complex than that.

“We are in this world where it’s one-size-fits-all in terms of when we expect people to feel 'energized' to work and be productive, and that could really be misaligned with people’s individual circadian rhythm,” MacDonald said.

If we agree that a person’s chronotype influences cardiovascular health and that it also influences energy levels and productivity, she continued, then perhaps we can extend that ideology to being physically active and exercise, identifying when an individual’s internal clock is most primed to exercise.

“This brings me back to the well-documented variability in how one responds to exercise, across a broad range of health outcomes — could another potential factor be that we are not considering the timing of exercise?” MacDonald proposed. “If we have an evening chronotype come into the laboratory in the morning, we have them exercise, and then measure their physiological responses throughout the day, it might be that we see blunted or minimal cardiovascular benefit because this is an additional stressor, and the timing is misaligned with when their body is most 'primed' to exercise.”

There are a handful of ways to figure out someone’s chronotype: validated questionnaires and 24-hour ambulatory circadian monitoring (ACM), which is the most accurate but also the most expensive and time-consuming.

Malone’s study has two parts, the first is to classify individuals as morning, intermediate or evening chronotypes using questionnaires, the less expensive and less invasive techniques. Next, participants will wear ACM devices that measure their physical activity, core temperature and light exposure for seven consecutive days (including sleep) to, hopefully, confirm their chronotype, previously measured via questionnaires.

In the second part of the study, participants will be asked to complete a series of lab visits, which includes two experimental sessions where they will perform the same exercise routine at different times of the day: In the morning from 6-7 a.m. and on a different day in the evening from 6-7 p.m.

MacDonald and Malone hypothesize that one’s physiological responses to exercise will differ by exercise timing and that these differences will be contingent on chronotype. Specifically, MacDonald said she thinks they may observe blunted or less favorable responses for evening chronotypes following the morning exercise session and likewise for the morning chronotypes following exercise in the evening.

“(If our hypotheses are supported with the study results,) that would provide some pretty interesting information to explore on a larger scale. It could be hugely important in terms of exercise prescription and optimizing cardiovascular health benefits,” she said. “Not only do you want people to feel good and energized when they exercise, but you also want exercise to elicit a beneficial effect. If we can continue to chip away at this very complex picture and provide additional insight toward individually tailoring exercise prescription, perhaps by considering something as simple as one’s chronotype, I think that would be pretty cool.”

MacDonald’s other graduate students are busy working on their own studies as well.

Garrett Reuben, who started working on his doctorate this fall at the University of Wisconsin-Madison, is working to publish his master’s thesis research that investigated the influence of chronotype on cardiovascular, metabolic, and sleep health. There were noticeable differences in physical activity patterns, quality of dietary intake and sleep health, specifically greater sleep debt, which is an indicator of circadian misalignment.

“With classes during the week, students who aligned more with the evening chronotype, took advantage of the weekend, staying up later and sleeping longer. Essentially, their schedules Monday through Friday were much different from their weekends, when they could spend their 'free time' doing what they wanted, and importantly, when they wanted,” MacDonald said.

“Collectively, these were young and healthy college-aged students, so we didn’t find drastic differences in cardiovascular health markers, but we did observe differences in key health behaviors – sleep, physical activity, and diet – all of which contribute to changes in cardiovascular health,” she said. “This begs the question, are these the early behavioral differences, that overtime, manifest and progress to the marked health differences we see in chronotypes with middle-aged and older adults?”

If true, then with Malone’s study, we hope to better understand the interaction between chronotype and exercise timing so exercise can be more precisely prescribed to prevent and treat elevated cardiovascular risk among evening chronotypes, she continued.

Erica Schaefer, a former research fellow at the U.S. Army Research Institute of Environmental Medicine (USARIEM) and now second-year graduate student, recently received the Doctoral Student Research Grant from the National Strength and Conditioning Association Foundation for her dissertation work studying women tactical athletes, a demographic we know very little about, MacDonald said. These tactical occupations include women currently serving in the military, law enforcement, correctional officers, firefighters and emergency medical service workers.

Schaefer’s study, “Assessing Total Health, PErformance, and Nutritional Attributes in Female Tactical Athletes: the ATHENA Study,” seeks to characterize the overall health and lifestyle of women in these tactical professions, the physiological, psychological and occupation-specific demands they face as well as the key attributes that are predictive of readiness and performance.

Schafer’s long-term goal for this line of research is to contribute to the development of evidence-based sex-specific interventions, policies and training programs that enhance the success and health of tactical women.