Does insomnia contribute to heart disease, or is heart disease a contributing factor for insomnia?
Maintaining a healthy heart is central to preventing heart or cardiovascular disease (CVD), and our sleep is part of this strategy.
The Centers for Disease Control and Prevention (CDC) advise that "most adults need at least 7 hours of sleep each night."
But what links sleep to our hearts?
Experts have previously identified associations between poor sleep and heart disease.
Earlier this year, Medical News Today featured a study that identified a significantly greater risk of death from heart disease in people who had high blood pressure together with insomnia and stress.
So, what can another study add to the picture?
Most population studies look for associations, but they cannot pinpoint cause and effect. While it may make sense to many people that poor sleep causes poor heart health, there is little data to confirm which is the chicken and which is the egg.
A new study aims to do just that.
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Using Mendelian randomization
Susanna Larsson, an associate professor of cardiovascular and nutritional epidemiology at the Karolinska Institute in Stockholm in Sweden, and Dr. Hugh Markus, a professor of stroke medicine at the University of Cambridge in the United Kingdom, published their analysis this week in the journal Circulation.
What sets this study apart from others is the use of Mendelian randomization (MR). This method uses genetic markers to investigate whether a risk factor can cause a particular disease.
A person inherits genetic markers, which will not change during their lifetime. Therefore, the technique works under the assumption that a disease cannot modify these markers. This allows scientists to look at causation.
Earlier this year, a large scale genetic analysis using data from 1.3 million people identified 956 genes across 202 locations in our genome that have a link to insomnia.
Larsson looked at a group of genetic variants called single nucleotide polymorphisms (SNPs) from this dataset and any potential ties to heart health. SNPs are mutations that only change one base in a particular genetic sequence.
This analysis allowed her to measure the odds of cardiovascular disease risk associated with an individual's genetic propensity toward insomnia.
Larsson used publicly available data from large association studies of people with various forms of heart disease.
Specifically, her analysis included 184,305 individuals from a coronary artery disease study, 394,156 from a heart failure study, 588,190 from an atrial fibrillation study, and 438,847 from a stroke study. The majority of the study participants were of European ancestry.
In those individuals with the insomnia genetic variants, the odds of developing coronary artery disease were 12% higher than in those without the SNPs. For heart failure, the odds were 16% higher, and for all types of stroke combined, they were 7% higher.
There was no change in the odds of developing atrial fibrillation in people with the insomnia SNPs.
"In conclusion, this MR study indicates that liability to insomnia is associated with a modest increased risk of CVD," Larsson and Dr. Markus write in the paper.
Strengths and weaknesses
Larsson and Dr. Markus explain in the paper that one of the strengths of their analysis lies in the large dataset that they used.
On the other hand, one of the shortcomings is that while MR can help identify cause and effect, in this case, the data do not prove that insomnia itself is the cause of CVD.
"Thus, our findings may not necessarily mean that insomnia itself is a cause of CVD. We cannot rule out that there are other causal pathways leading to insomnia that cause CVD," they explain.
Another weakness is that the SNPs that the researchers used in this analysis only account for 2.6% of the genetic variance that occurs in insomnia, meaning that they only contribute a small amount to the likelihood of a person developing insomnia.
In fact, Larsson did not know which of the study participants who had heart disease also experienced insomnia.
Nevertheless, this study adds to the wider picture linking sleep habits to heart health.
"It's important to identify the underlying reason for insomnia and treat it."