The next time the boss finds you leaning back in your chair, feet up, eyes shut, tell her that you're napping for medical purposes.

The next time the boss finds you leaning back in your chair, feet up, eyes shut, tell her that you're napping for medical purposes.

Science won't definitively back you up yet, but the evidence is mounting that a short afternoon nap, for an otherwise well-rested, healthy person, is good for the heart. "I love to nap," says Dr. Robert Downey III, chief of sleep medicine at Loma Linda University Medical Center in Southern California. "I recommend napping."

Among his fellow researchers, it's still controversial whether napping has cardiovascular benefits. Early studies of possible heart benefits of siestas in Mediterranean and Latin American countries, where short afternoon naps are typical, have had mixed results. But a recent large study of 23,000 people in Greece, published in the Feb. 12 Archives of Internal Medicine, showed a 37-percent reduction in heart attacks among people who napped at least three times a week for a minimum of 30 minutes. That study was the first to weed out sick and sedentary nappers and control for physical activity and diet, which might have colored results of other studies showing no benefit.

Now a study, in the Oct. 15 online edition of the Journal of Applied Physiology, offers a clue about why a nap might be good.

The new study tested nine healthy volunteers who did not usually nap. After sleeping for four hours the night before, each was hooked up to check for blood pressure under three conditions. In two sessions the volunteers relaxed, one time standing and once lying down, but didn't sleep. In the third session, the subjects fell asleep for no more than an hour.

The researchers found a significant drop in blood pressure when the volunteers slept, but not when they merely relaxed. And the drop in blood pressure when they napped occurred in that sleepy window of time right before falling asleep, not during the nap itself.

It's the brief period of anticipation of the coming snooze where cardiovascular benefits take place. Just lying awake, even if relaxed, doesn't do the trick, says Greg Atkinson, chronobiologist with the Research Institute for Sport and Exercise Sciences at Liverpool John Moores University, and an author of the study.

That makes sense to Downey. "Anything you gain in (a restful state) would be magnified by going to sleep," he says. But he cautions that for a nap to result in increased alertness, it probably should last 20 to 30 minutes.

"With a light nap, you get that soothing, biological benefit, but you don't have to fight your way back to consciousness," he says. That's because brain waves slow considerably as you get drowsy, more so when you sleep. But after about 30 or 40 minutes, the brain goes into still-deeper sleep, and waking can make a person feel more groggy than rested.

Younger children, and adults who are sleep deprived, go into deeper states of sleep quicker, so a short nap for them might be more likely to result in a groggy awakening. And insomniacs are generally advised not to nap, Downey says, to give them a better shot at sleeping at night.

Inadequate sleep — seven to eight hours is recommended for the average adult — as well as disorders such as sleep apnea are pervasive in American society, and people who feel sleepy throughout the day should talk to their physician, says Michael Twery, director of the National Center on Sleep Disorders Research at the National Heart Lung and Blood Institute. "If sleepiness is affecting what they do, controlling their lives, or if they wake up with morning headaches, maybe they need a medical evaluation," he says.

But for healthy people who get a good night's sleep, the human circadian clock is set to want a brief nap in the early afternoon. It feels good, and it just might be good for the heart, to indulge that post-lunch loss of energy. Go ahead. Put your feet up and close your eyes.