Scientists have found that the time of day when you exercise might be an important factor in lowering all-cause and cardiovascular mortality .
Exercise is good, but what about timing?
The value of physical activity for health and longevity is unquestionable , even if some doubts remain about its optimal composition and quantity. However, little is known about the effect of daily exercise timing.
This new study published in Nature Communications is one of many to utilize the trove of health information accumulated in UK Biobank, a huge open repository of health data. In addition to more conventional stuff, such as bloodwork and dietary patterns, UK Biobank also contains data on about 100,000 UK residents who had agreed to wear sophisticated accelerometers constantly for 7 days. This data enables researchers to precisely determine exercise intensity and timing. It also covers physical activity that is not part of structured exercise, such as carrying groceries or walking stairs, which is something that most related studies are missing.
Physical activity is associated with a massive reduction in mortality
The researchers divided the participants into four groups by prevalent timing of MVPA (moderate-to-vigorous physical activity): morning, evening, midday-afternoon, and mixed. The mean follow-up period was seven years.
First, the data confirmed yet again that physical activity is associated with massively reduced mortality. Interestingly, and largely in line with other recent studies, the association increased rapidly between 0 and 150 minutes of MVPA per week and plateaued at around 200 minutes per week. This was true for all three types of mortality considered: all-cause, cardiovascular, and cancer. For cardiovascular mortality, a massive four-fold reduction was observed. The effect was weaker, though still remarkable, for cancer mortality.
Midday beats mornings and evenings
The main result, however, was that people who had the lion’s share of their MVPA either in the morning or in the evening seemed to enjoy a smaller reduction in all-cause and cardiovascular mortality than those who exercised mostly during midday-afternoon or mixed hours. Even in models adjusted for numerous potentially confounding variables, such as age, sex, ethnicity, socioeconomic status, education level, diet, smoking, alcohol intake, sleep quality, and total MVPA volumes, the midday-afternoon and mixed hours groups showed 28% and 26% reduction in cardiovascular mortality, respectively, compared to the morning group. The evening group results were largely similar to those of the morning group. The difference for all-cause mortality was significant as well but not for cancer mortality.
These associations held even after numerous sensitivity analyses and were more pronounced for older people, males, people with existing cardiovascular conditions, and less physically active people.
What are the mechanisms?
Speculating about possible mechanisms behind their results, the researchers mention the circadian differences in cardiometabolic reaction to physical exercise that were uncovered by previous studies. For example, one study showed faster recovery of systolic blood pressure after exercise in the late afternoon than in the early morning . This would be consistent with the stronger effect shown in people with existing cardiovascular conditions. Another study found that post-meal walking improves glucose control more effectively than morning/afternoon walking does .
Like all populational studies, this one cannot establish causation and is prone to contamination by numerous potentially confounding variables. It also has design limitations, such as the short, one-week duration of accelerometer wearing. This might be enough to capture weekly activity patterns but not long-term changes in physical activity. On the other hand, the study implements robust safeguards and presents convincing results that should be explored further.
To our knowledge, this large cohort study provides the first evidence that MVPA is associated with lower risks of all-cause, CVD, and cancer mortality regardless of the time of day. Another interesting finding was that the midday-afternoon and mixed MVPA timing groups, as compared to the morning group, showed substantially decreased all-cause and CVD mortality risks, but not cancer mortality. The associations between MVPA timing and mortality risk were independent of sociodemographic factors, lifestyle, comorbidities, sleep duration, sleep midpoint, and total MVPA volume. These findings were robust to multiple testing corrections and sensitivity analyses. In addition, the observed protective effects of MVPA timing were more pronounced among the elderly, males, less active individuals, or those with preexisting CVDs.
While the effects of daily timing of sleep and feeding are drawing most of the attention, the timing of physical activity is not as well-researched. The results of this study suggest that when we exercise might be just as important as how we exercise. However, more research is needed to confirm this effect and uncover the mechanisms behind it.
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 Feng, H., Yang, L., Liang, Y. Y., Ai, S., Liu, Y., Liu, Y., … & Zhang, J. (2023). Associations of timing of physical activity with all-cause and cause-specific mortality in a prospective cohort study. Nature Communications, 14(1), 1-10.
 DiPietro, L. (2001). Physical activity in aging: changes in patterns and their relationship to health and function. The Journals of Gerontology Series A: Biological Sciences and Medical Sciences, 56(suppl_2), 13-22.
 Qian, J., Scheer, F. A., Hu, K., & Shea, S. A. (2020). The circadian system modulates the rate of recovery of systolic blood pressure after exercise in humans. Sleep, 43(4), zsz253.
 DiPietro, L., Gribok, A., Stevens, M. S., Hamm, L. F., & Rumpler, W. (2013). Three 15-min bouts of moderate postmeal walking significantly improves 24-h glycemic control in older people at risk for impaired glucose tolerance. Diabetes care, 36(10), 3262-3268.
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