There's new evidence that daytime naps may be linked to a lower risk of heart attack or stroke, but only if they're limited to a few times a week.

The research, published Monday in the journal Heart, is based on data collected from nearly 3,500 people living in Switzerland.

"We looked at healthy adults and found that people who take occasional naps — once or twice a week — had a lower risk for cardiovascular disease compared to people who were not napping at all," said Nadine Häusler, an internist at University Hospital of Lausanne, and lead author of the new research.

Häusler and her colleagues tracked the participants for five years. All were between 35 and 75, basically healthy without any evidence of heart disease, and none were overly sleep-deprived.

The frequency of napping varied. More than half (58 percent) said they never took a daytime siesta, while about one in 10 said they nodded off almost daily.

About one in five participants hit what the researchers found to be the napping sweet spot: one to two times per week.

It was that occasional nap frequency that was linked to a 48 percent lowered risk for heart attack, stroke or heart failure.

Nap length did not appear to influence the findings, and included anything from a quick, five-minute catnap to an hour-plus snooze. Because the study was observational, it cannot prove cause and effect.

"It could be that that these people who nap once to twice a week are those who make napping a priority, because they know they don't sleep enough during the week," said Céline Vetter, an assistant professor at the University of Colorado Boulder who studies circadian rhythms and sleep disruption. Vetter was not involved with the current research.

How napping may influence heart health is unclear.

"Our best guess is that a daytime nap just releases stress from insufficient sleep," Häusler said.

Indeed, a large study of healthy adults living in Greece published in 2007 found those who napped at least three times a week had a lower risk of fatal heart attacks. The strongest benefits were found in working men, and researchers theorized at the time that naps helped reduce stress.

This does not mean physicians should start writing prescriptions to nap for optimal heart health, mainly because there's no way to know what "dosage" is best.

"What is the timing, duration and frequency of the naps? Do we count in a 5 min 'dozing-off' as a nap?" wrote Yue Leng, an epidemiologist studying sleep behavior at the University of California San Francisco, in an editorial published alongside the new study.

"We don't really know much about napping," Leng told NBC News. "We have a lot to learn."

Häusler agreed. "It's really important that other studies confirm these findings," she said.

Much more is known about the benefits of a good night's sleep.

"Every physiological function we look at — from metabolism to the immune system — is all embedded in how much you sleep," Vetter said.

Sleep deprivation also drives up the risk for obesity, a known risk factor for heart disease.

But when it comes to daytime naps, there appears to be a fine line between when they may be helpful, and when they may do harm.

A study published in February suggested that people who catch up on missed sleep during the week by napping on weekends tend to snack more, increasing their risk for excess weight gain.

In older adults, frequent naps may be a sign of an underlying problem, like Alzheimer's disease.

"If older adults report taking a lot of long, extended naps during the day, that might be an alarm for clinicians," said Leng.

Proven ways to reduce heart disease risk include a diet rich in fruits, vegetables and heart-healthy oils, not smoking, keeping weight and blood pressure to healthy levels, and frequent exercise.

Meanwhile, the American Academy of Sleep Medicine recommends most adults get at least seven hours of shuteye a night for a variety of health benefits.

A good bedtime routine can help, according to sleep experts, who recommend these tips:

Get up at the same time every day, even on weekends or during vacations.

Keep your bedroom room at a comfortable, cool temperature.

Limit exposure to bright light in the evenings, and avoid caffeine and alcohol before bed.

Turn off electronic devices at least 30 minutes before bedtime.

Exercise regularly and maintain a healthy diet.

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