Don’t Snooze on Good Sleep: Older Adults With Fragmented Sleep Have Increased Risk of Mortality
Despite the fact that we spend up to one-third of our lives sleeping, the science of why we need so much of it, and why some people don’t do it as well as others, remains somewhat of a mystery. While adequate sleep is good, more sleep is not always better — the risk of mortality and cardiovascular disease increases both in those who sleep too little and too much. Although sleep duration is a simple and commonly measured metric, many other factors reflect how well we snooze, including the quality, regularity, continuity, and depth of our sleep. While we know that poor sleep increases the risk of heart disease, we haven’t fully understood yet how a lack of solid shut-eye impacts mortality — and what these differences are between men and women.
In a recent study published in the European Heart Journal, Shahrbabaki and colleagues looked at three large cohorts of older adults to better understand how sleep duration and fragmented sleep patterns affect cardiovascular health and mortality. Using sleep data from over 8,000 participants, the researchers uncovered that waking up often in the night could be as detrimental to health and longevity as short sleep duration — especially in women. With these sex-specific sleep results, this Australia-based research team suggests that simple sleep modifications could be a practical step towards improving heart health — and with it, increasing both the healthspans and lifespans of older adults.
Disrupted Sleep and Heart Health: What’s the Link?
One way that researchers quantify sleep quality is by the arousal burden, which tracks how many times you wake up in the night. A higher arousal burden suggests greater sleep fragmentation, which is linked to poor sleep quality and increased emotional and physical fatigue during the day. This unconscious waking in the night — also known as cortical arousal — is a natural part of sleeping, even if you don’t realize that you wake up. These minor sleep disturbances can occur from environmental factors like traffic noise and bright lights, from sleep disorders, like sleep apnea or periodic limb movement disorder, which causes jerking of the legs during sleep, or they can be completely spontaneous. Whether or not someone fully wakes from and remembers a cortical arousal can vary; however, most people will find themselves increasingly fatigued the next day after having fragmented sleep.
Besides causing daytime exhaustion, cortical arousals are thought to impact heart and vascular health. In the short-term, like right after an arousal occurs, heart rate and blood pressure will elevate — although benign when it happens every once in a while, consistently higher heart rate and blood pressure can lead to hypertension. In the long-term, chronic sleep fragmentation can disrupt the cardiovascular system’s circadian rhythm — our body’s 24-hour internal clocks. When the heart’s circadian rhythm is impeded night after night, the risk of heart attacks, heart failure, and cardiovascular disease increases.
Women Shoulder the Greater Burden of Poor Sleep
While previous research has solidified that inadequate (less than 6 hours) or excessive (more than 9 hours) sleep increases the risk of cardiovascular-related deaths, this is the first study to look at sex-specific associations between sleep arousal burden and mortality.
To better quantify the sleep data, Shahrbabaki and colleagues created a sleep ‘arousal index’, combining the nightly arousal burden (which includes frequency and duration of unconscious waking) with total sleep time. The research team looked at data from three groups of older adults totaling over 8,000 participants: the Osteoporotic Fractures in Men Study (MrOS), the Study of Osteoporotic Fractures (SOF), and the Sleep Heart Health Study (SHHS). Briefly, the participants’ ages ranged from 64 to 83 years, the majority were white, and approximately half were overweight and had health conditions, including high blood pressure and diabetes.
Using overnight sleep trackers that record brain waves, oxygen levels, heart rate, breathing, and eye and leg movements, the researchers found that the average sleep time was about 6 hours per night. People with a lower arousal burden also tended to have longer sleep durations, unsurprisingly. Those with higher arousal burdens were more likely to be older, obese, ex-smokers, diabetic, asthmatic, or have a history of health disorders, including sleep apnea, hypertension, or heart failure.
During follow-up periods ranging from 6 to 11 years, approximately 20-25% of the study participants passed away. Shahrbabaki and colleagues found that those in the highest arousal burden groups had greater mortality, both from cardiovascular-related events or from any cause. Women had a greater risk of dying associated with fragmented sleep, with an arousal burden of 6.5% (meaning, 6.5% of one’s sleep was disrupted by cortical arousals) increasing both all-cause and cardiovascular-related mortality. Although the frequency of cortical arousals was lower in women than in men, the relationship with mortality was stronger in women.
In the two cohorts that included women, those in the SOF group with an arousal burden over 6.5% had more than double the risk of dying from cardiovascular disease, while women in the SHHS group had a 60% increased risk. In cases of all-cause mortality, the SOF and SHHS women were at a 50% and 20% increased risk of dying from any cause, respectively.
Conversely, the men in this study had a less clear view of the relationship between arousal burden and mortality. While the MrOS cohort showed that cardiovascular-related mortality risk increased by 30% in men with an arousal burden of over 8.5%, the SHHS group found the same increased 30% risk for all-cause mortality — but not vice versa. As the men’s results were not consistent between cohorts, the research team finds the effect of arousal burden on mortality to be stronger in women.
Is Reducing Sleep Arousals the Key to Better Heart Health?
The researchers are unsure why the arousal burden affects women’s cardiovascular health and mortality so much more than men. As one of the authors, Professor Dominik Linz, speculates, "The triggers causing an arousal or the body's response to arousal may differ in women compared to men. This may explain the relatively higher risk of cardiovascular death in women. Women and men may have different compensatory mechanisms for coping with the detrimental effects of arousal. Women may have a higher arousal threshold and so this may result in a higher trigger burden in women compared to men."
Although the researchers still aren’t entirely sure why these sex-specific differences in sleep fragmentation and cardiovascular health exist, this study does add to the evidence that sleep quality and duration are essential for heart health and longevity. Although some of the risk factors associated with greater arousal burden in this study, like age and health history, cannot be changed, some factors are modifiable, including weight and current health conditions.
The researchers are hopeful that altering some of these factors may improve both sleep quality and cardiovascular health outcomes, but more studies are needed to determine this definitively. As Professor Linz summarizes, “Whether this will translate into lower risks of dying from cardiovascular disease warrants further study...We need to advise our patients to take care of their sleep and practice good sleep hygiene. Measures to minimise noise pollution during the night, lose weight and treat sleep apnoea could also help to reduce the arousal burden."
Shahrbabaki SS, Linz D, Hartmann S, Redline S, Baumert M. Sleep arousal burden is associated with long-term all-cause and cardiovascular mortality in 8001 community-dwelling older men and women [published online ahead of print, 2021 Apr 20]. Eur Heart J. 2021;ehab151. doi:10.1093/eurheartj/ehab151
Wang C, Bangdiwala SI, Rangarajan S, et al. Association of estimated sleep duration and naps with mortality and cardiovascular events: a study of 116 632 people from 21 countries. Eur Heart J. 2019;40(20):1620-1629. doi:10.1093/eurheartj/ehy695