Taking a daytime nap increases the risk of a stroke, according to new research. It can be an early warning sign of high blood pressure - which reduces oxygen to the brain, say scientists.
The findings add to evidence that nodding off in the chair has potentially serious implications. They are based on 360,000 Britons who were tracked for an average of around eleven years.
Corresponding author Professor E Wang, of Xiangya Hospital Central South University, said: "These results are especially interesting since millions of people might enjoy a regular, or even daily nap."
The Chinese team analysed genetic, lifestyle and health information on 40 to 69 year olds in the ongoing UK Biobank study. Those who said they usually nap were 12 and 24 percent more likely to develop high blood pressure or have a stroke, respectively.
Under 60s who often did so were a fifth (20%) more prone to high blood pressure than peers of the same age - with risk halving for over 60s. Prof Wang and colleagues combined observational information with Mendelian randomisation - a technique that uses mutations linked to a specific risk factor.
Participants were divided into groups - "never or rarely," "sometimes" or "usually." If napping frequency increased by one category high blood pressure risk rose 40 percent. The symptomless condition affects more than a quarter of adults in the UK - around 14.4 million. Around a third have not been diagnosed.
It can lead to blocked vessels - triggering a heart attack or stroke. Napping was also linked to a genetic propensity for high blood pressure - known medically as hypertension. In June the American Heart Association's new Life's Essential 8 cardiovascular health score added sleep duration as the 8th metric for measuring optimal heart and brain health.
Prof Michael Grandner, of Arizona University, who co-authored the updated guidelines, said: "This may be because, although taking a nap itself is not harmful, many people who take naps may do so because of poor sleep at night. Poor sleep at night is associated with poorer health, and naps are not enough to make up for that.
"This study echoes other findings that generally show that taking more naps seems to reflect increased risk for problems with heart health and other issues."
Earlier this year a study of more than 1,000 elderly people in the US found those who napped daily were 40 percent more likely to develop dementia. Not getting seven to eight hours sleep at night has been linked to a host of other illnesses including diabetes and cancer.
It can lead to diminished brain performance and, in the long term, greater risk of health conditions. These include heart disease, stroke, and diabetes. The study is in Hypertension.
