Ask anyone why they have sex, and they're most likely to say they do it for fun. And while that's certainly a valid reason for a good romp between the sheets, sex has other physiological effects on the body.
And yes, there are people who study this for a living.
Two of the most famous sex researchers were William Masters and Virginia Johnson. They studied what they termed the sexual-response cycle, which involves excitement, plateau, orgasm and resolution.
Though each of those phases look different for each individual (and even sometimes each sexual encounter), sex has roughly the same effect on everyone. So if you really need another reason to have sex (or you're just curious about what's really going on in your brain and the rest of your body), keep reading for five ways having sex affects your body.
1. Sex Can Help Relieve Pain and Lower Stress
The chemicals your brain releases during sex can also help relieve pain and lower your stress levels. A 2010 Princeton study found that rats who were more sexually active were less anxious, as sex stimulated cell growth in the hippocampus (an area of the brain associated with anxiety regulation).
While there's not a one-to-one correlation between rats and humans, there's plenty of anecdotal evidence to suggest this holds true for people (maybe you even have an anecdote of your own). And the endorphins released during sex act as natural painkillers. So Betty Dodson, PhD, a sex educator in private practice in New York City, recommends having sex or masturbating to help relieve menstrual cramps.
2. Sex Helps You Bond With Your Partner
When you orgasm, your body releases oxytocin, a chemical responsible for those warm, fuzzy feelings and post-orgasm bliss. Oxytocin is also essential for creating a strong bond between sex partners. Basically, the couple that orgasms together, stays together (or has a higher likelihood of staying together). Just make sure you're not attaching yourself to a partner who's incompatible. Oxytocin doesn't distinguish between true love and a one night stand on it's own.
3. Sex Puts Your Body in Overdrive
You already know that sex is exciting, but your brain and body know it (and show it), too. Sex causes your heart rate, blood pressure and breathing rate to elevate. Blood is routed to the penis, vagina and nipples, making them hyper-sensitive to sensory input.
The rest of your skin may also become more sensitive to touch as well, and your pupils will often dilate. And when you climax, muscle tension and blood-vessel engorgement reach a peak, too. Some orgasms also result in a grasping-type muscular reflex of the hands and feet.
4. Sex Improves Your Heart and Immune System
Because your body (especially your heart) is working harder during sex than it would at rest, sex builds cardiac stamina. As your heart rate increases from about 70 beats per minute to 150, your heart is getting a great workout (see #5 for more). And not only is sex good for your heart, but studies have shown that it can also boost your immune system.
For example, a St. Joseph's University study from psychologists Carl Charnetski and Francis Brennan, Jr., found that students who had sex at once or twice a week had higher levels immunoglobulin A (IgA), which is one of the body's first immune responses against cold viruses.
5. Sex Can Be a Pretty Decent Workout
All that activity between the sheets can help you burn excess calories and tone up some major muscle groups. Depending on length and intensity of the sex session, you can burn anywhere from 85 to 250 calories. It's certainly not enough to replace your daily workouts, but every little bit helps keep you active and your heart healthy.
And if you and your partner are adventurous enough, some of those more challenging positions can even help strengthen and sculpt your abs, thighs and arms.
What Do YOU Think?
So why do you have sex? For fun? Of course! But how many of these other reasons did you know? Does it change your view on sex at all? Have you heard of any other effects that sex has on the body? Share your suggestions, insights and questions in the comments section below!
Rachel Grice is a contributing fitness editor for Livestrong.com and a certified yoga instructor. Previously, she was the science section editor and titling associate for several other sections of Demand Media Studios, including fitness, nutrition and technology. She completed her undergrad education at the University of Southern California and has worked for Men's Health, FitPregnancy and People magazines.