kiss
© GettyCouples who share a passionate kiss this Valentine's Day will enjoy sensations of relaxation.
The reason that kissing feels so pleasurable is that sparks a surge of hormones in our brains, according to new research.

Couples who share a passionate kiss this Valentine's Day will enjoy sensations of relaxation and excitement because of a complex series of chemical processes, as well as their love for their partners.

The study showed that women need more than just a kiss to experience the same chemical high as men - with additional features such as a romantic atmosphere of dimmed lights and mood music also required.

Wendy Hill, professor of psychology at Lafayette College, Pennsylvania began the research to find out why the mundane physical activity of rubbing lips can elicit such a gratifying emotional response.

Her team tested the levels of two hormones, cortisol and oxytocin, in 15 couples before and after holding hands and kissing.

They found that kissing reduced the levels of cortisol, a stress hormone, in both sexes. But levels of oxytocin, a hormone linked to social bonding that they expected to be boosted by kissing, only rose among the men.

The scientists have since replicated the tests in more intimate settings, to see if the less-than-alluring environment of the university health centres where the original research was carried out hampered women's hormonal surge.

The final results will be presented at the annual conference of the American Association for the Advancement of Science in Chicago this week.

"This study shows kissing is much more complex and causes hormonal changes and things we never thought occurred," said Prof Hill.

"We tend to think more about who we are kissing and how it feels, yet there are a lot of other things happening."

It is not clear how kissing provokes such hormonal reactions, but some scientists believe they are triggered by the exchange of pheromones - chemicals our bodies release to attract sexual partners - in the saliva.

This interaction may also have health benefits. Helen Fisher of Rutgers University, New Jersey, said: "If you share your germs with somebody, you're boosting your internal defence system."

This is not the first research to analyse the physical effects of kissing. In 2007 British scientists measured the brain and heart activity sparked by passionate kissing, but found it was less intense that the stimulation produced by eating chocolate.

Romantic love has also been shown to have a close link to neurological activity, with scans showing that it has similar effect to cocaine on our brains.