Short sleep duration can cause more heart attacks

Scientists find that short sleeping hours can independently increase risk of heart attacks. Thus, sleep is a potentially modifiable cardiovascular risk factor.

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Executive Summary

In a brilliant new study, scientists ‘proved’ that short sleeping hours increases the risk of heart attacks.

Sleeping for 7 to 8 hours was the best.
Sleeping for less than 6 hours increased the risk of heart attacks by 20%.
Sleeping for less than 5 hours increased the risk of heart attacks by 54%.

Even in people genetically predisposed to getting heart attacks —where one assumes nothing can be done— sleeping 7 to 8 hours led to an 18% reduction in heart attacks compared to sleeping less than 6 hours.

Read below how the scientists used a new technique called Mendelian randomisation to find out this causal relation between sleep and heart attacks.

In a remarkable new study, researchers found that sleeping less than 6 hours increased the risk of heart attacks. But, unlike all the earlier studies, this study showed a causal relation between the two: low sleep hours actually ’caused’ more heart attacks.

The study

Researchers at the Massachusetts General Hospital, U.S.A. and the University of Manchester, U.K., took the records of 461,347 participants between the ages of 40 and 69, who had no heart disease.

They analyzed their medical records, self-reported sleep habits, as well as the genetic information. The participants were then followed up for seven years.

The results

The results were published in the Journal of the American College of Cardiology in Sept 2019.

As it has been observed in earlier studies, this study also showed increased risk of heart attacks with short sleep. It found that, compared to those who slept between 6 to 9 hours, those who slept less than 6 hours had 20% more risk of heart attack during the seven year study period.

Among the people who had short sleep, those with frequent insomnia had 30% higher risk and those with difficulty rising from bed had 81% higher risk, as against the average 20% higher risk mentioned above.

Interestingly, sleeping too long was also not good. Those who slept more than 9 hours a night were 34% more likely to get a heart attack compared to those who slept between 6 to 9 hours.

The people who slept 5 hours a night had 52% higher chance of getting a heart attack than those who slept 7 to 8 hours. And, those who slept 10 hours a night had 100% higher (double) risk of getting a heart attack than those who slept 7 to 8 hours.

Confounding

The results above were commensurate with many papers published earlier. The problem is, and was, that these results can not show any causality.

In other words, the results can only tell you that short sleep hours go hand in hand with an increased risk of heart attacks. It cannot say whether reduced sleep causes more heart attacks, or more heart disease leads to reduced sleep.

In fact, there could be many other factors that might be causing both. For example, unhealthy lifestyle, such as smoking, might be causing reduced sleep, and simultaneously causing heart attacks. These other factors are called Confounding Factors. They confuse, or confound, the conclusion.

Randomisation

To find out if short sleep indeed increases heart attacks, an ideal way would be to compare two groups of people, who are identical in all respects except their duration of sleep. Such groups will be difficult to find in real life.

People who sleep proper hours might be more disciplined than the people who sleep less hours. Maybe, they also have less stressful life, better work habits, and better food choices.

There are various statistical techniques that help in removing the effects of such confounding factors, or confounders. However, they can never remove the confounding effect completely. As a result, there is always some uncertainty in drawing concrete conclusions about which factor causes what.

Mendelian randomisation

There is a wonderful new technique, called Mendelian randomisation, which can remove this effect fully. It was first proposed in 1986. So, it is quite new to the research world.

It uses the science of genetics. For example, there are dozens of genes that lead to short sleep. These genetic variants cause reduced sleep ‘independent of’ any other habit of an individual.

So if you take two groups of people, one with the presence of a genetic variant causing short sleep, and the other with its absence, the difference in the heart attack rates of these groups can tell you whether short sleep actually ’causes’ more heart attacks.

Genetic predisposition

The study discussed above looked at 27 different genetic variants that cause short sleep. People with these variants had less sleep compared to their normal colleagues, by sheer nature of their genes.

Compared to healthy sleepers, those who had short sleep due to genetic variants had 130% higher risk of heart attacks. Note that those who had short sleep, with and without genetic variants, had an average 20% higher risk of heart attacks, as mentioned earlier. Thus, having a genetic variant for short sleep significantly increased the risk of heart attacks.

However, those who had a genetic variant for short sleep but still slept adequately had an 18% reduced risk compared to their colleagues who had a genetic variant and slept short. In a rough way, we can say that the healthy sleepers with genetic variants had only 89%, and not 130%, higher risk than healthy sleepers with no genetic variants.

The researchers concluded that even in people who were highly predisposed to heart attacks due to having genetic variants for short sleep, sleeping for 7 hours offered a cardio-protective effect.

Dr Nicola Montano from the University of Milan, Italy wrote an editorial to this article. He said that “this causal link is the most important result for the entire preventive cardiology community, as it finally confirms and corroborates the hypothesis already inferred by other observational studies of a causal relationship between sleep duration and risk of MI.

The discussion above, for medical professionals: Sleeping <6 hours daily can up heart attack risk.

Why short sleep causes heart disease

The paper did not try to find the reasons why short sleep causes increased heart attacks. However, many other articles have given various hypotheses and data about that.

As per an article by Centers for Disease Control and Prevention (CDC), U.S.A., during normal sleeping hours, your blood pressure goes down. If you have sleep problems, the blood pressure stays higher for a longer period of time. High BP is a risk factor for heart disease.

Lack of sleep also worsens blood sugar control, which in turn damages blood vessels. Not getting enough sleep increases hunger, which leads to unhealthy weight gain. Obesity increases heart disease.

An article by Harvard Medical School says that ‘sleep-deprived people have higher blood levels of stress hormones and substances that indicate inflammation, a key player in cardiovascular disease’.

Researchers have found that sleep–deprived bodies produce high levels of toxic products called cytokines, such as C-reactive protein, interleukin-6, and tumor necrosis factor–alpha. These are markers of inflammation in the body and are linked to heart disease and diabetes.

Short sleep impairs the brain reward systems that regulate energy consumption, judgment, and food choices. So, sleep–deprived people eat fewer vegetables and more processed, sweet, and fatty foods.

People may avoid exercise, if they feel fatigued due to lack of sleep.

A review article in the journal Current Cardiology Reviews discusses epidemiological studies that have shown relationships between sleep deprivation and high blood pressure, heart disease, and diabetes.

Signs of short sleep

Thus, short sleep is a health hazard. Excessive daytime sleepiness is considered a clear sign of sleep deprivation. Read on this website: Are you suffering from excessive daytime sleepiness?

There are other, subtle signs of short sleep. Read on this website: 14 signs of sleep deprivation that you may miss.

Actionable tips

  1. Sleep is a potentially modifiable cardiovascular risk factor.
    Sleep 7 to 8 hours a day, especially if you are facing any risk factor for heart attacks, such as obesity, stress, family history, diabetes, bad eating habits, etc

Image by pranav digwal from Pexels

First published on: 28th March, 2020

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