How Much Does It Matter When You Exercise?

The headlines make us dizzy. “For a longer life, afternoon exercise may be best,” says the Washington Post. “Morning Workouts May Lower Your Risk of Heart Attack and Stroke,” according to Verywell Health. However, BBC offers yet another view with a headline saying, “Best exercise time may differ for men and women.” Despite the wild variations in these headlines (and many more), all of them depend on the same presumption – that there is a best time of day for exercise. But is that really so? How much does it really matter when you exercise?

The short answer is not so much.

From the UK Biobank

The latest fuel for such headlines comes from a study this week in Nature Communications. Hongliang Feng and colleagues used data from 92,139 persons in the UK Biobank for whom they had both valid measurements of physical activity and mortality outcomes. They found an association between afternoon physical activity and lower mortality versus morning activity. This observation comes from an impressive 638,825 person-years of data. But it is still observational data. So it’s simply not possible to draw conclusions about causal relationships between the timing of physical activity and mortality. The authors say this clearly, deep within the discussion of their findings.

Moreover, they note that physical activity “at any time of day” predicts lower mortality. So really, the headlines should be just do it. There’s no compelling reason here to believe that if we time our exercise perfectly, we will surely live longer.

Sensational Clicks

The sensational headlines have only one purpose – to make you look, so the media can sell its ads. It may well be true that some times of day can be slightly better than others for moderate or vigorous physical activity. But it’s unlikely that what works for one will work for all.

So the best guide for finding a good time to work out is not some mouse study or a huge biobank study. It is your own understanding of what works best for you and the rigors of your daily life.

At any time of day we can do it safely, physical activity makes our life and our health better.

Click here for the study in Nature Communications and here for yet another recent study in PNAS. For perspective on the metabolic basis for speculation about exercise timing, click here.

Morning, painting by Mikalojus Konstantinas Čiurlionis / WikiArt

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February 24, 2023