Is exercising late in the morning associated with a reduced risk of cardiovascular disease?

Is it better to exercise in the morning or later in the day to reduce the risk of cardiovascular disease? This is a question that Dutch researchers have tried to answer in a study of 86,657 participants in the UK-Biobank cohort, aged 62 on average. Participants’ physical activity data was collected at the start of the study using a wrist-worn triaxial accelerometer over a 7-day period. Six years after the start of the study, 3,707 cardiovascular events had been reported. Participants who exercised late in the morning had a 16% lower risk of a coronary event and a 17% lower risk of a stroke, compared to those who exercised at another time of day.

These effects were particularly pronounced in women. In contrast, most of the favourable associations of morning physical activity disappeared when the researchers analyzed data from men only. This difference remains unexplained and raises the possibility that a confounding factor may be the cause. Do women who exercise in the morning have better lifestyle habits, unrelated to physical exercise, such as better diet?

Previous studies had shown a favourable association between morning physical activity and better cardiometabolic health, both for obesity (see here, here and here), type 2 diabetes, and hypertension. However, a number of studies have shown completely opposite results. For example, a recent study in Brazil indicates that for hypertensive men, evening exercise was more effective than morning exercise in restoring the heart rate and lowering blood pressure. Additionally, a Swedish study of men with type 2 diabetes indicates that high-intensity interval training (HIIT) performed in the afternoon was more effective than morning exercise in improving blood sugar levels. It should be noted that these last two intervention studies are of the “randomized and controlled” type, a study design that provides a relatively high level of scientific evidence, even if these studies were carried out with a small number of participants.

Further studies will be needed to better understand the chronoactivity phenomena, but regardless of whether it is done in the morning, afternoon or evening, it is well established that physical exercise is beneficial for cardiovascular health, mental health and overall health.