(CNN) -- Yoga instructor Sadie Nardini and her husband got an early start on their New Year's resolution: In December, the New York couple decided to have sex every day for the entire month.
Nardini and her husband, a professional photographer, initially decided to have sex like bunnies in the hopes that all the activity might help them overcome his-and-her bad habits: cigarettes and chocolate, respectively.
And indeed, the nightly trysts did help. But they also found, unexpectedly, that frequent sex made them feel better in other ways, too.
Nardini says they both slept better and had more energy, and she didn't get a cold or the flu all month as she usually does in the winter.
"Sex doesn't seem at first glance to be the cure for what ails you, but there's so many health benefits of having more sex," Nardini says. "Anyone can be better served by having more sex."
In fact, the experiment was so successful, the couple plans to have daily sex in January, too.
Penis penetration gives most benefits
Researchers have long known that not only is sex fun (when done with the right person, of course), but that people who have frequent sex tend to live longer and have healthier hearts and lower rates of certain cancers.
These studies also show that men with an active sex life have healthier sperm, and sexually active women have fewer menopause symptoms.
The possible health benefits of sex are spelled out in a soon-to-be-published article in the Journal of Sexual Medicine.
The editor-in-chief of that journal, Dr. Irwin Goldstein, says that when you read about the physical benefits of sex, "you can't help but say, 'Holy God! Sexual activity is a very important thing to do. Human beings were really meant to do this.' "
Goldstein is also the director of sexual medicine at Alvarado Hospital in San Diego, California.
In the article, which reviews several decades' worth of studies on sex and health, researcher Stuart Brody concludes that penile-vaginal intercourse -- and not other types of sexual activity -- confer the most health benefits.
More sex, more calm?
For example, 5 years ago, Brody, now a professor of psychology at the University of the West of Scotland, asked 46 people to give a stressful speech to a nasty audience.
His team measured parameters such as blood pressure and levels of stress hormones while the participants prepared for the speech and directly after giving it.
He found that the study subjects who'd had intercourse in the 2 weeks preceding the speech had markedly lower blood pressure and lower levels of stress hormones.
The same wasn't true for people who'd participated in other forms of sexual activities. Brody says he's not surprised by the findings.
"Evolution is not politically correct," Brody says. "Evolution strongly rewards behaviors that increase the likelihood of successful gene propagation, and only one behavior -- penile-vaginal intercourse-- is potentially reproductive."
More reasons to have sex
But sex researcher Beverly Whipple, who wrote an article in 2003 titled "The Health Benefits of Sexual Expression," disagrees, saying that other types of sexual activity are also associated with physical health benefits.
She adds that it's not entirely clear whether sex makes people healthier, or whether healthy people tend to have more sex.
"It's a chicken-egg situation," says Whipple, professor emerita of psychology at Rutgers University, in New Jersey. "We can't definitively say there's a cause-and-effect relationship between sex and better health."
While the underlying reasons might be unclear, here's a list of health benefits that people who have frequent sex enjoy.
1. A longer life. In a British study, men who had intercourse at least twice a week lived longer than men who had sex less than once a month.
A U.S. study had similar findings, and a Swedish study examining the sex lives of 70-year-olds found that men who died before their 75th birthday had ceased having sexual intercourse at earlier ages.
The Swedish study didn't find that women lived longer if they had sex more frequently, and neither did a study in North Carolina.
However, in the North Carolina study, women who reported enjoying sex more lived longer than those who didn't report enjoyment.
2. A healthier heart. In a British study, people who had intercourse twice a week or more were less likely to have heart attacks and other fatal coronary events.
Those who had sex less than once a month had twice the rates of fatal coronary events, compared with those with the highest frequency of intercourse.
3. Lower blood pressure. In a study published in the journal Biological Psychology, people who had sex more often tended to have lower diastolic blood pressure, or the bottom number in a blood pressure reading.
Brody's experiment, in which more sexually active study subjects had markedly less dramatic blood pressure spikes when they were put under stress, also supports the benefit.
4. Lower risk of breast cancer. A French study found that women who have vaginal intercourse not at all or infrequently had 3 times the risk of breast cancer, compared with women who had intercourse more often.
5. Lower risk of prostate cancer. A Minnesota study found that men who'd had intercourse more than 3,000 times in their lives had half the prostate cancer risk of those who had not.
While it's not clear why this would be true, studies have found that men who had more intercourse tended to have better prostate function and eliminated more waste products in their semen.
"These differences could conceivably impact prostate cancer risk," Brody writes in his article.
6. Pain relief. Whipple and others have conducted studies suggesting that more sexual activity helps relieve lower back pain and migraines.
7. A slimmer physique. A study of healthy German adults revealed that men and women who had sex more frequently tended to be slimmer than folks who didn't have as much sex.
Sex burns 50 to 60 calories per encounter, Whipple says, so sex 3 times a week for a month would burn about 700 calories -- or the equivalent of jogging about 7 miles.
8. Better testosterone levels. A group of men being treated for erectile problems saw greater increases in testosterone when, along with the treatments, they had frequent sex.
Specifically, men who had sex at least 8 times per month had greater increases than those who had sex less than 8 times per month.
9. Fewer menopause symptoms. Menopausal women in Nigeria experienced fewer hot flashes when they had sex more frequently.
Brody says this may be because sexual activity helps regulate hormonal levels, which in turn affect the symptoms of menopause.
10. Healthier semen. In 3 studies, men who had frequent intercourse had a higher volume of semen, a higher sperm count and a higher percentage of healthier sperm, compared with men who tended to participate in other sexual activities. With reports from John Bonifield, CNN.