Morning people had more heart attacks, not fewer

The early riser has a good reputation. Waking at six is treated as evidence of discipline, and staying up late as evidence of the opposite, with health assumed to follow.

An analysis of the Sleep Heart Health Study, published in Sleep Health, tested that assumption against heart attacks in 4,578 adults over a decade. Circadian preference has emerged as a potential determinant of cardiovascular health, yet its role in myocardial infarction risk remains underexplored, its authors write, and previously nobody had checked whether the early end of the spectrum was really the safe one. It found the early risers doing worse than the people in the middle.

The night owls did worse too. Being at either end was the problem.

What a chronotype actually is

Circadian rhythms include some of the physical, mental, and behavioral changes an organism experiences over a 24-hour cycle, governed by what the National Institute of General Medical Sciences calls a biological clock. The master clock is a large group of nerve cells that form a structure called the suprachiasmatic nucleus.

That clock does not run at the same phase in everyone. Some people’s runs early, some late, and the position is largely inherited.

In this study, chronotype was derived using the standard Munich Chronotype Questionnaire (MCTQ) algorithm applied to self-reported bedtimes and wake times, and categorized as morning, intermediate, or evening type. It is a well-used instrument, and it is still a questionnaire.

What the Sleep Heart Health Study found

The researchers analyzed 4578 adults aged 40 years or older from the Sleep Heart Health Study, and incident myocardial infarction events were adjudicated during a mean follow-up of 10.6 years. Adjudicated means a committee checked the records, rather than taking anybody’s word for it.

Compared with intermediate chronotypes, both morning and evening types exhibited significantly higher myocardial infarction risk after full adjustment. That is roughly 50% higher for the early risers and 40% for the late ones, against the people in the middle.

The authors flag the counterintuitive half themselves: notably, the morning group did not confer the lowest risk.

Their conclusion is a U-shape rather than a slope. Chronotype was independently associated with long-term myocardial infarction risk, with intermediate chronotype showing the lowest risk.

Why sleep apnea changes the whole picture

Split the cohort by whether people had obstructive sleep apnea and the finding stops being general.

These associations were substantially stronger among participants with obstructive sleep apnea, whereas no significant association was observed in those without obstructive sleep apnea.

Obstructive sleep apnea causes your airway to collapse or become blocked during sleep, and normal breathing starts again with a snort or choking sound. It is common, it is treatable, and doctors diagnose sleep apnea based on medical and family histories, a physical exam, and sleep study results.

So the honest summary is narrower than the headline number. In people breathing normally at night, chronotype tracked nothing. In people whose breathing was already disturbed, an extreme body clock came alongside a substantially higher rate of heart attack.

What the overnight sleep studies add

Most chronotype research rests entirely on questionnaires, which makes it hard to separate a body clock from the sleep it produces. A late type who sleeps five hours and a late type who sleeps eight are not the same person.

This cohort has something better. Every participant had an overnight sleep study, and the models adjusted for demographic, lifestyle, cardiometabolic, and polysomnography-derived sleep parameters.

That means the chronotype association is not simply short sleep wearing a different label. Whatever is going on survived adjusting for how long and how well people actually slept, measured in a laboratory.

What 4,578 people cannot settle about body clocks

This is observational, so it reports that a pattern travels with heart attacks. It cannot show that a body clock causes one.

Chronotype also travels with things that are hard to strip out. Shift work, light exposure, social schedule, mood and alcohol all shift with it, and the two extremes differ from the middle in more ways than the clock.

The evening estimate is the weaker of the two: its range runs down to essentially no difference, so that half of the U-shape rests on thinner ground than the morning half.

The paper is paywalled, so the authors’ own limitations section was not available for this piece.

What to do about a morning or evening chronotype

Nothing, on the strength of this. There is no evidence here that moving your body clock changes your heart risk, and chronotype is not a habit you can simply decide to drop.

The part worth acting on is the sleep apnea. That is where the association concentrated, it is common, and unlike a chronotype it can be treated. Heavy snoring, waking with a gasp, or feeling wrecked after a full night in bed are all reasons to raise it with a doctor.

The reputational story about early risers was never based on heart attacks. This study is a reason to stop repeating it.

If you do have sleep apnea, one measurement predicts repeat cardiac events better than BMI does. Waist-to-height ratio sorted a large trial population that BMI left undifferentiated.