Female insomniacs may be more at risk of a premature death, research suggests.
Sleep is critical to our wellbeing, with insufficient shuteye linked to everything from heart disease and diabetes to depression and infertility.
When it comes to heart health specifically, too much or too little sleep may cause cardiovascular complications.
To better understand the risks, scientists from Maastricht University in the Netherlands analysed the sleeping habits of 8,001 adults over one night.
Results reveal the women who experienced "unconscious wakefulness" – spontaneously rousing from sleep – most often or for longer periods of time were almost twice as likely to die over the next six to 11 years.
Many people wake in the night, often in response to noise, pain or a change in temperature.
"A common trigger for nocturnal arousals is obstructive sleep apnoea when breathing stops and the arousal system ensures the activation of our body to change our sleep position and to reopen the upper airway," said study author Dr Dominik Linz.
"Another cause of arousals can be noise pollution during the night by, for example, night-time aircraft noise.
"Depending on the strength of the arousal, a person might become consciously aware of the environment, but often that is not the case.
"Typically, people will feel exhausted and tired in the morning because of their sleep fragmentation, but will not be aware of the individual arousals."
Irrespective of the cause, these arousals are known to affect a person's heart rate, blood pressure and circulation, particularly "when frequent", the Maastricht scientists wrote in the European Heart Journal.
Sleep disruption has also been linked to inflammation, internal stress and a "pro-thrombotic state", to name a few complications.
A lack of shuteye can also leave people feeling tired and irritable the next day, which could prompt them to make unhealthy lifestyle choices.
The Maastricht scientists analysed data collected by sleep monitors worn during a single night by the participants of three studies.
Overall, the female participants had a lower "arousal burden" than the men.
Nevertheless, the women whose arousal burden took up more than 6.5% of their night's sleep were nearly twice as likely to die from heart disease over the next six to 11 years than their low arousal burden counterparts.
Their risk of dying from any cause was 40% higher, the results show.
This remained true after adjusting for other factors that could influence a person's heart health and premature death risk, like their underlying fitness, weight and smoking status.
The men whose arousal burden accounted for more than 8.5% of their night's sleep were found to be up to 30% more likely to die from heart disease or any cause in one of the three studies.
The same was not true in the remaining two studies, however.
"It is unclear why there is a difference between men and women in the associations, but there are some potential explanations," said Professor Linz.
"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."
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Age, weight and the severity of any sleep apnoea influence a person's arousal burden, of which the latter two can be modified.
Sleep apnoea can be treated with a CPAP machine, which gently pumps air into a mask worn during shut eye. Losing weight, if necessary, and nodding off on your side can also help.
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"For me as a physician, a high arousal burden helps to identify patients who may be at higher risk of cardiovascular disease," said Professor Linz.
"We need to advise our patients to take care of their sleep and practice good sleep hygiene."
Sleep hygiene habits include establishing a set bed and wake time; making your bedroom sleep-friendly – like with a comfortable mattress and temperature; and winding down before lights off – such as with yoga, a relaxing book or soothing music.
"Measures to minimise noise pollution during the night, lose weight and treat sleep apnoea could also help to reduce the arousal burden," added Professor Linz.
Undetected fat in the arteries can also affect a person's body clock, according to Professor Borja Ibáñez from Madrid's National Centre for Cardiovascular Research, who was not involved in the study.
The Maastricht scientists have stressed the three studies' participants were generally older, average age 74, and white. The results may therefore not apply to the general population.
The sleep effects of any medications were also not accounted for. In addition, the participants' shut-eye was measured on just a single night.
"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," said Professor Ibáñez.
"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."
Watch: How to improve your sleep