Taking naps of longer than one hour can cause heart disease - and may even trigger an early death, warns a new study.
Researchers found that long naps in the afternoon are associated with a 30 percent greater risk of dying young, and a 34 percent higher likelihood of cardiovascular disease compared to not napping.
The findings fly in the face of perceived opinion that lying down for a snooze is a harmless activity, with an afternoon "siesta" considered in some countries to improve productivity.
But the latest findings by Chinese scientists, due to be presented at the European Society of Cardiology Congress, suggest that drifting off for more than 60 minutes could be risky.
Study author Dr. Zhe Pan, of Guangzhou Medical University, said: "Daytime napping is common all over the world and is generally considered a healthy habit.
"A common view is that napping improves performance and counteracts the negative consequences of 'sleep debt'. Our study challenges these widely held opinions."
Previous studies of the link between daytime naps and death or cardiovascular disease have produced conflicting results. The new research also took account of the duration of night-time sleep as well.
The Chinese team summarised available evidence to assess the relationship between napping and the risks of dying and cardiovascular disease.
A total of more than 313,000 participants from more than 20 studies were included in the analysis. Around 39 percent of the participants took naps.
The analysis found that long naps - classified as more than 60 minutes in duration - were associated with a 30 percent greater risk of any cause of death and 34 per cent higher likelihood of cardiovascular disease compared to not napping.
When night-time sleep was taken into account, long naps were linked with an elevated risk of death only in those who slept more than six hours per night.
Overall, naps of any length were linked with a 19 per cent elevated risk of death, according to the findings.
The connection was more pronounced in women, who had a 22 per cent greater likelihood of death with napping compared to no napping, and older participants, whose risk rose by 17 percent with naps.
Short naps - less than 60 minutes - were not risky for developing cardiovascular disease.
Dr. Pan said: "The results suggest that shorter naps - especially those less than 30 to 45 minutes - might improve heart health in people who sleep insufficiently at night."
The reasons why napping affects the body are still uncertain, said Dr. Pan, but some studies have suggested that long snoozes are linked with higher levels of inflammation, which is risky for heart health and longevity.
Other research has connected napping with high blood pressure, diabetes, and poor overall physical health.
Dr Pan added: "If you want to take a siesta, our study indicates it's safest to keep it under an hour.
"For those of us not in the habit of a daytime slumber, there is no convincing evidence to start."
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