Is there anything nicer than falling asleep on a lazy afternoon, with a snuggly pet and a half-read book?

Giving in to this simple pleasure too often might be a sign worth heeding, however, with a huge new study concluding regular, frequent napping is correlated with a higher risk of hypertension and stroke.

An analysis of 358,451 anonymous UK Biobank records revealed the link, suggesting it may be more than coincidence.

Being a correlational study, the figures don't necessarily imply the fault lies with the naps themselves. It's entirely possible poor sleep patterns is the problem, and those brief moments of daytime rest mightn't be enough to protect us from the health deficits that occur as a result.

"These results are especially interesting since millions of people might enjoy a regular, or even daily nap," said anesthesiologist E Wang of Xiangya Hospital Central South University in China.

Previous studies have noted a potential link between daytime napping and hypertension and, by extension, one of its major complications, stroke. In fact, a single day's siesta can coincide with a surge in the risk of hypertension and stroke.

Both these conditions are serious: hypertension, diagnosed through persistent high blood pressure, is a major cause of early death, and stroke can cause serious, long-term disability, and death.

To expand further on previous studies, the researchers tapped into the Biobank to conduct a study that included Mendelian randomization – the effect genetic variations have on an outcome to determine causality – and observations that took place over long periods of time.

The database has information on over 500,000 UK residents aged between 40 and 69 from the years 2006 and 2010, who regularly provided samples and updates on their health. There was also a survey on daytime napping conducted by some of the Biobank participants; this survey took place four times between 2010 and 2019.

From the Biobank data, the researchers excluded any individuals who already had high blood pressure, or had a stroke, prior to the commencement of the study. This left 358,451 people whose health information contributed to the study, including 50,507 incidents of hypertension and 4,333 incidents of stroke.

This wide sample revealed some fascinating information. For example, most of the regular nappers were male, smoked, drank daily, had lower education and income levels, and reported both insomnia and snoring.

Higher napping frequency was also positively associated with the genetic predisposition towards hypertension.

These regular nappers had a 12 percent higher risk of hypertension than seldom- and never-nappers, and a 24 percent higher risk of stroke. And this risk was higher for younger participants below the age of 60, whose hypertension risk was 20 percent, compared to 10 percent for over 60.

Increased nap frequency, reported by around a quarter of the participants, is also a cause for concern. Increasing napping frequency by just one category on the survey – for example, from never to sometimes – increased the risk of hypertension by 40 percent.

Rather than one causing the other, the naps and the hypertension might both be symptoms of the same underlying problem.

"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," said clinical psychologist and sleep expert Michael Grandner of the University of Arizona, who was not involved in the research.

"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."

That doesn't mean a causal link should be ruled out. Scientists have previously recorded that blood pressure can rise following a nap; that may play a role in the risk increase in stroke in daytime nappers, although another mechanism may be responsible too. At any rate, it certainly seems that further research is warranted.

"Our study," the researchers wrote in their paper, "along with previous clinical studies, suggests that further examination of the mechanistic basis of the association between a healthy sleep pattern, including daytime napping, and cardiovascular disease is necessary."

The research has been published in Hypertension.