The researchers analyzed data from 86,657 individuals in the UK Biobank. Their average age was 62, and they ranged in age from 42 to 78. The majority of individuals, 58{b574a629d83ad7698d9c0ca2d3a10ad895e8e51aa97c347fc42e9508f0e4325d}, were women. Accelerometers recorded participants’ physical activity over a period of 7 days.

Dr. Siddiqi expressed some concern about the study’s fitness tracking, noting that “[t]he participants were followed for only 7 days, and in a manner that involved an ‘intervention’ — wearing an accelerometer, which they otherwise may not do — and we always need to be cognizant of how these interventions affect participant behavior, i.e., the Hawthorne Effect.”

“The real question is whether a 1-week tracking period is a fair representation of habits throughout the 6 to 8-year follow-up period,” he said.

Dr. Arciero, however, did not consider this a problem, saying that “[b]ased on a previous research study, a 7-day physical activity accelerometer measurement is considered a representative time period to assess a person’s overall physical activity level.”

The lead author of the study is Dr. Gali Albalak of Leiden University Medical Center in the Netherlands. Even though people’s activity could have changed over time, she told MNT, the accelerometer data “enables objective measurement of physical activity without any recall or other forms of bias that play a role in questionnaire-based physical activity information.”

In any event, added Dr. Siddiqi, “[i]n general, given that the study population was largely middle-age[d] to older adults with presumable ‘formed’ behavioral habits and patterns, there isn’t a strong suggestion that these habits would change significantly over the follow-up period.”

After 6 years, 2,911 study participants developed coronary artery disease (CAD), and 796 participants had had a stroke.

Overall, people who exercised at 11 a.m. were 16{b574a629d83ad7698d9c0ca2d3a10ad895e8e51aa97c347fc42e9508f0e4325d} less likely to develop CAD, and 17{b574a629d83ad7698d9c0ca2d3a10ad895e8e51aa97c347fc42e9508f0e4325d} less likely to have a first stroke of any kind. They were also 21{b574a629d83ad7698d9c0ca2d3a10ad895e8e51aa97c347fc42e9508f0e4325d} less likely to have a first ischemic stroke, compared to the study’s reference group.

People who exercised early in the morning lowered their CAD risk by 11{b574a629d83ad7698d9c0ca2d3a10ad895e8e51aa97c347fc42e9508f0e4325d}.

When the researchers tabulated women and men separately, it turned out that women benefited considerably more from morning exercise. By itself, the benefit of morning exercise for men was not statistically significant.

Women who exercised in the early or late morning reduced their risk of CAD by 22{b574a629d83ad7698d9c0ca2d3a10ad895e8e51aa97c347fc42e9508f0e4325d} and 24{b574a629d83ad7698d9c0ca2d3a10ad895e8e51aa97c347fc42e9508f0e4325d}, respectively.

“In support of this large prospective study,” added Dr. Arciero, “a recent 3-month exercise intervention study published from our laboratory with a group of middle-aged, healthy-weight women and men showed that women exercising in the morning reduced systolic and diastolic blood pressure to a significantly greater amount (-10{b574a629d83ad7698d9c0ca2d3a10ad895e8e51aa97c347fc42e9508f0e4325d} vs. -3{b574a629d83ad7698d9c0ca2d3a10ad895e8e51aa97c347fc42e9508f0e4325d}) compared to women exercising in the evening.”

“Thus, our data also supports morning exercise in women to improve cardiovascular health, and therefore lower incident risk for CVD,” he argued.

Dr. Albalak noted: “We were not able to explain this difference in our finding[s]. One of the possible explanations could be that the etiology of cardiovascular disease is different in men and women.”

“So, more research is needed,” said Dr. Albalak, “before we can make any recommendations for men or women. However, we see our research as a first step in the right direction.”

Dr. Albalak noted that “[a]s this was an observation[al] study, we cannot say anything about the causal effect of physical activity timing on our [cardiovascular] health.” Still, she pointed out that the study’s findings align with her group’s previous work.

“We already were able to identify that morning physical activity was associated with metabolic health — BMI, glucose levels, and insulin resistance — in a group of 207 healthy but sedentary older adults,” she said.

“Also, physical activity is, just like food intake and exposure to light, an important ‘Zeitgeber,’ or circadian clock trigger,” said Dr. Albalak.

“This means that with these behavioral aspects, we can calibrate our biological clock and the circadian rhythms with each other and our environment [day and night cycle of the earth],” she added.

“From other research, we know that eating after 8 p.m. or exposing ourselves to bright light at night can have detrimental effects on our biological clock. We hypothesize that being physically active in the morning is the most appropriate timing to correctly set your clock.”

Dr. Siddiqi noted:

“For the majority of people, we are simply trying to introduce the habit of physical activity, and so any time of day is better than nothing. As a general rule, though, I try to make exercise promotion simple and flexible to increase adoption, recognizing that timing of exercise is a fine adjustment that has practical aspects and limitations.”