The reason that kissing feels so pleasurable is that it 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.

根据新的研究,接吻让人感到愉快的原因是它刺激大脑大量地分泌荷尔蒙.

因为一系列复杂的化学过程和他们彼此之间的爱,情人节热吻的情侣们可以感受到放松和兴奋.

研究表明女性要达到和男性同样高的化学反应,一个吻是不够的--还需要例如昏暗夜里一个浪漫的气氛和曼妙音乐.

Wendy Hill,宾夕法尼亚拉法叶大学心理学教授,开始研究为什么摩擦嘴唇这样的身体活动可以引起愉悦的情感反应.

她的团队测试了15对情侣牵手和接吻后皮质醇和催产素两种荷尔蒙的分泌水平.

他们发现接吻在两性中都会产生皮质醇--一种压力荷尔蒙.但是催产素,这种和社会结合密不可分的荷尔蒙,只有男性在接吻时才会产生.

科学家在更亲密的环境中重复试验,来验证当初在大学健康中心测试得出的不浪漫的环境会阻碍女性荷尔蒙分泌的结论是否成立.

最后结果将在本周芝加哥举行的美国科学进步协会年会上亮相.

Hill教授说,"这个研究表明接吻过程复杂得多,它改变了荷尔蒙,产生我们从来没想过的物质."

"然而在这些事情潜移默化发生的时候,我们更关心我们在和谁接吻,感觉如何."

尽管接吻是如何引发荷尔蒙的变化还不清楚,但是一些科学家相信荷尔蒙的变化是唾液中交换的性激素--我们身体释放出来吸引异性的化学物质--的触发器.

这个相互作用可能对健康也有好处.新泽西州Rutgers大学的Helen Fisher说,"如果你和他人分享你的细菌,你的体内防御系统就会加强."

这不是接吻这种身体影响的第一个试验.在2007年英国科学家测量了热吻后的大脑和心脏活动,但是发现它比吃巧克力的刺激更轻松.

观察表明,浪漫的爱情和精神活动也有联系,它和可卡因对我们大脑的影响类似.