Taking midday naps often may significantly reduce the risk of death from coronary heart disease, according to a new study by researchers from the Harvard School of Public Health (HSPH) and the University of Athens Medical School (UAMS) in Greece.

Subjects who took a nap regardless of the frequency and duration were one third less likely to die from heart attack or stroke than those who did not, the researchers reported in the February 12, 2007 issue of The Archives of Internal Medicine.

But some experts quickly warned that people should not rely on taking a nap to reduce their risk of heart disease.  The results of the current study did not suggest that napping caused reduction of death risk from coronary events. Other factors may be involved.

Earlier studies were inconsistent. Some studies found afternoon napping was linked with reduced risk of heart disease, but some suggested otherwise.

The current study meant to establish an association between taking a nap and coronary mortality based on data from "a sizable cohort with a high frequency of napping and information on potentially confounding variables including reported comorbidity, physical activity, and diet."

The study involved 23,681 men and women living in Greece from the Greek European Prospective Investigation into Cancer and Nutrition [EPIC] cohort, who had no history of coronary heart disease, stroke or cancer at enrolment or at the start of the study.

Androniki Naska, lead author of the study, lecturer of hygiene and epidemiology in UAMS, and senior author Dimitrios Trichopoulos, professor of cancer prevention and epidemiology at HSPH followed the participants for an average of 6.3 years for their napping habit and coronary mortality.

They found that nappers regardless of how often and or how long they took naps were 34 percent less likely to die from heart disease and stroke than those who did not.

Those who regularly took naps, defined as three times a week for an average of at least 30 minute each time, had a 37% lower coronary mortality than those who did not take siestas.

Even occasional nappers seemed to benefit from napping.  They were 12 percent less likely to die from coronary heart disease than those who did not take any nap, but the risk reduction was not significant.

"Siestas are common in the Mediterranean region and several Latin American countries and those countries also tend to have low mortality rates of coronary heart disease," according to news release by Harvard School of Public Health.

The authors believed that an afternoon siesta in a healthy person may help relieve stress, which would otherwise adversely affect incidence of and mortality of coronary heart disease.

The association between napping habits and risk of coronary mortality was particularly stronger in men who worked at the time of enrolment. It was not significant among the retirees.   The possible benefit of taking a nap was unknown among women as few women died during the study, making the association impossible.

When explaining why siesta did not help the retirees, but working men, the researchers suggested that this was because they had different levels of stress to cope with.

The message for the public to take home is, have some nap time if you can, Trichopoulos was cited as saying. But a scientist associated with foodconsumer.org said that it is too early to count on napping to reduce the risk of coronary mortality and napping may not definitely offer the benefit.

The current finding was apparently contradictory to some other studies.   In the International Journal of Epidemiology 2000; 29:429-437, Hannia Camposa and Xinia Siles from Harvard School of Public Health and University of Costa Rica reported that those who had siesta for long and often were more likely to have myocardial infarction (heart attack), indicating that taking a nap could actually be a sign of cardiac problems.

The authors found that those who napped longest and most often were 50 percent more likely to have MI, compared to those with the lowest siesta frequency.   They concluded that "our data suggest that the practice of daily siesta is associated with increased risk of MI."

Campos and Siles’ results were questioned by some researchers who said that the afternoon nap is commonly practiced in China and other countries, but the risk of coronary heart disease is low in those countries. For instance, in Greece, the siesta practice was linked with lower risk of CHD among the population involved in manual labor.

Campos responded to a Letter to Editor in the International Journal of Epidemiology 2001; 30:183 saying that sedentary behavior and obesity are common in the population they studied in Costa Rica, but not as common in China and Greece, which makes the difference in the association between napping and risk of heart disease.

Citing earlier studies, Campos pointed out that those who reported that they often or almost always took naps were at elevated risk of stroke and CHD and were more likely to be diabetic, overweight, hypertensive and hypercholesterolaemic.

Campos said that their findings were based on a population-based case-control study design. And "the cases were recruited within a week of their first MI and the questions asked referred to the year prior to their MI," Campos write in his response.

The apparent contradiction between the two studies may be caused by some hidden difference between the healthy statuses in the two populations, a scientist affiliated with foodconsumer.org suggested.   He agreed with some others that people with certain conditions may tend to take a nap because of their physiological needs whereas the healthy people may take a nap just as a way to save energy or take a break.   He noted that in China, middle school students regularly take a midday nap before starting their afternoon school work.

The lower risk of heart disease in the Mediterranean region may be better explained by the diet they use, which is known to benefit cardiovascular health.   In explaining why the seemingly protective effect was not found among retirees, the foodconsumer.org scientist said it is possible that retirees were already in a position to have a high risk of coronary problems. Because of this, napping may not help them much if it does have any effect at all.

In any case, the current study did not mean to establish a causal relationship between afternoon napping and risk of coronary mortality, meaning that taking a nap would not necessarily reduce the risk of coronary heart disease.   Those who seek ways to protect against heart disease and other cardiovascular disease should focus more on other lifestyle attributes such as diet and physical exercise.