Previous research has shown that sleep duration, either too short or too long, is associated with increased risks of death from cardiovascular or other causes. However, until now, it was unknown whether there was also a link with the arousal burden (a combination of the number of arousals and their duration) during a night's sleep and the risk of death.
In a collaboration between a team led by associate professor Mathias Baumert from the School of Electrical and Electronic Engineering at the University of Adelaide (Australia) and professor Linz, researchers looked at data from sleep monitors worn overnight by men and women taking part in one of three studies: 2782 men in the Osteoporotic Fractures in Men Sleep Study (MrOS), 424 women in the Study of Osteoporotic Fractures (SOF), and 2221 men and 2574 women in the Sleep Heart Health Study (SHHS). The average ages in the studies were 77, 83 or 64 years, respectively. The participants were followed up over a period of several years, which ranged from an average of six years (SOF) to 11 years (MrOS).
After adjusting for factors that could affect the results such as total sleep duration, age, medical history, body mass index (BMI) and smoking habits, the researchers found that women had an arousal burden that was lower than men. However, those who had an arousal burden that accounted for more than 6.5 per cent of their night's sleep had a greater risk of dying from cardiovascular disease than women with a lower arousal burden: double the risk in SOF and 1.6 times the risk in SHHS. Their risk of dying from all causes was also increased by 1.6 times in SOF and 1.2 times in SHHS.
Taking the women from both studies together, those with an arousal burden of more than 6.5 per cent had a 12.8 per cent risk of dying from cardiovascular disease, nearly double that of women of a similar age in the general population who had a risk of 6.7 per cent. The risk of dying from any cause was 21 per cent among women in the general population, which increased to 31.5 per cent among women in the two studies with an arousal burden of more than 6.5 per cent
Men with an arousal burden accounting for more than 8.5 per cent of their night's sleep had 1.3 times greater risk of dying from cardiovascular disease (MrOS) or any cause (SHHS), compared to men with lower arousal burdens, but findings for increased risk of death from any cause in MrOS or cardiovascular disease in SHHS were not statistically significant.
When the researchers looked at all the men in both studies, those with an arousal burden of more than 8.5 per cent had a risk of 13.4 per cent and 33.7 per cent of dying from cardiovascular disease or any cause, respectively, compared to the risk in the general population of men of similar ages of 9.6 per cent and 28 per cent, respectively.
Professor Linz said, "It is unclear why there is a difference between men and women in the associations, but there are some potential explanations. 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."
He said that older age, BMI and the severity of sleep apnoea increase arousal burdens. "While age cannot be changed, BMI and sleep apnoea can be modified and may represent an interesting target to reduce arousal burdens. Whether this will translate into lower risks of dying from cardiovascular disease warrants further study. For me as a physician, a high arousal burden helps to identify patients who may be at higher risk of cardiovascular disease. 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."
Professor Baumert said, "In order to include assessment of arousal burdens into routine strategies for reducing the risk of cardiovascular disease, we need easily scalable, widely accessible and affordable techniques to estimate the duration and fragmentation of sleep and to detect arousals. Wearable devices for measuring activity and changes in breathing patterns may provide important information."
Limitations of the study include that it was conducted in older, mainly white people and so its findings cannot be extrapolated to other races or younger men and women. The researchers did not consider the possible effect of medications; monitoring of the participants' sleep was conducted on a single night and so does not take account of night-to-night variations. Finally, it can only show there is an association between greater sleep arousal burden and increased risk of death, not that sleep arousals cause the increased risk.
In an accompanying editorial, Professor Borja Ibanez, clinical research director at the Centro Nacional de Investigaciones Cardiovasculares Carlos III, Madrid (Spain), and colleagues, who were not involved in the research, write that a strength of the study is that the arousal burden was measured objectively with sleep monitors, rather than being self-reported by the participants. They point out that disruption of the body's natural circadian rhythm is known to be involved in the development of often undetected fat accumulation in arteries and this could be a possible mechanism for the increase in the risk of cardiovascular problems.
They continued, "Even though many knowledge gaps on the relationship between sleep and CVD [cardiovascular disease] remain to be studied in the coming years, this study provides solid evidence supporting the importance of sleep quality for a better CV health. Further evidence combining comprehensive sleep evaluation with biological sampling and long-term follow-ups will be desirable . . . What remains to be determined is whether an intervention aiming at improving sleep quality is able to reduce the incidence of CV events and mortality. While awaiting these trials, we wish you sweet dreams."