Morning in Paris

Early to Bed, Early to Rise … Has Links

“Morning workouts turbocharge the benefits of exercise,” says Psychology Today. A litany of headlines like this have been crossing our screen for weeks now. They are insistent. “The early bird gets the worm – and sees better workout results,” said People magazine. “This is the best time of day to work out if you want to lose weight.” Yes, but not exactly. A more honest description of the research is that it is an observation of links – associations – between early morning exercise and better outcomes in weight and diabetes.

There was no turbocharging, worm getting, or basis for promising better workout results if a person changes their routine exercise time to morning. None. Just the observation of an association.

Two Studies and a Bias of Familiarity

This association is not a new insight. Other studies have suggested an association between morning exercise and better weight or diabetes outcomes. What has been generating new headlines is the publication of two studies – both of them observational – that build upon a cultural bias extoling the virtue of early risers.

The first is a study in Obesity that is a cross-sectional examination of NHANES data on diurnal patterns of moderate and vigorous exercise and weight status. From a total of 5,285 persons, they found clusters of persons who exercised mainly in the early morning, midday, or evening. The morning cluster had less than a third as many people in it as either of the other two clusters. But for those relatively rare birds, the association between exercise and weight outcomes was significantly stronger. Hence the “turbocharging” claim.

But correlation does not equal causation. And correlation with a stronger correlation does not equal turbocharging.

The second study is also observational. It comes from a cohort of 93,095 persons in the UK Biobank. These investigators found an association between morning and afternoon physical activity with lower risk of type 2 diabetes. Physical activity in the evening had no such association – unless it was vigorous. Regardless of the time of day, vigorous physical activity had an association with lower diabetes risk.

Aphorisms That Resonate

The problem is twofold. First we have entrenched aphorisms that these observational studies tap into. The early bird gets the worm. Early to bed, early to rise, makes a person healthy, wealthy, and wise. People are ready to believe in the virtues of being a morning person.

Second is the astounding failure to remember that associations are not evidence of causality. Most of the headlines about research on diet and exercise spring from observational research. Links of early meals, early exercise, macronutrients, or food ingredients to health outcomes are only a signal to look closer. To do research designed to find evidence of causal relationships – or not.

So yes, early birds do indeed get worms and links of early morning activity to good health may ring true. But sometimes, the mere ring of truth is not the final word.

Click here for the study of weight outcomes and here for the study of diabetes risk.

Morning in Paris, painting by Pierre Bonnard / WikiArt

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September 25, 2023