It’s well established that sleep is vital to health and wellbeing, and that sleep deprivation has a host of adverse outcomes. What’s perhaps less well known is that excess sleep – commonly defined as more than 8–9 hours per night – can be too much of a good thing.
A large population study in the US adds new evidence to the mix, suggesting that 6–7 hours of sleep each night could be optimum for heart health.
Analysing data for more than 14,000 adults from the National Health and Nutrition Examination Survey between 2005 and 2010, researchers found that sleeping more or less than this increased risk of dying from heart attack, heart failure or stroke over a median 7.5-year follow-up.
The study was prompted by an increased prevalence of sleep disorders globally, says Kartik Gupta from Henry Ford Hospital, Detroit, lead author of the study to be presented at the 2021 American College of Cardiology conference scheduled for 15–17 May.
Gupta and colleagues divided participants into three groups based on their average sleep duration per night: less than six hours, six to seven hours or more than seven hours.
They collected data on levels of C-reactive protein (CRP), an inflammation marker associated with heart disease, and atherosclerotic cardiovascular disease risk scores (ASCVD). These are used to predict someone’s risk of having a heart attack or stroke or die from hardening of the arteries over the next 10 years, with a score under 5% considered low.
Adjusting for traditional risk factors such as age, hypertension and diabetes, analyses showed a U-shaped relationship between risk of death and sleep duration. CRP levels were highest in people who reported sleeping less and more than six to seven hours. The median 10-year risk was 4.6%, 3.3% and 3.3% for each respective sleep duration category.
“Participants who slept less than six hours or more than seven hours had a higher chance of death due to cardiac causes,” says Gupta. “ASCVD score was, however, the same in those who sleep six to seven hours versus more than seven hours.”
There are some caveats. First, respondents were limited to choosing one-hour blocks of sleep duration. The research was also based on self-reported sleep duration and didn’t include how well someone sleeps.
“It’s important to talk about not only the amount of sleep but the depth and quality of sleep too,” Gupta explains. “Just because you are lying in bed for seven hours doesn’t mean that you are getting good quality sleep.”
Sleep apnoea, for instance, wakes people up regularly and is associated with heart disease.
Further, sleep and inflammation data were only collected at one time point and the correlational data could have missed relevant information, so a cause-effect relationship can’t be established.
Finally, it bears noting that research is based on averages and everyone has their own unique sleep requirements, although sleeping less than six hours is generally not recommended.
Natalie Parletta is a freelance science writer based in Adelaide and an adjunct senior research fellow with the University of South Australia.
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