In the new study, published in the journal Psychoneuroendocrinology, researchers looked for biological evidence of health effects for the first time.
Women in long term relationships kept daily diaries for a week, including information on whether or not they had sex with their partner the previous night.
The study focused on telomeres - protective caps on the ends of DNA – that predict a person’s biological age.
It may also boost the immune system, increasing the production of natural killer cells which fight disease.
Experts found women who had regular sex had significantly longer telomeres - boosting their overall life expectancy.
Researchers from the University of California, San Francisco, said: “This is an important finding.
“It is possible the greater the frequency of sexual intimacy, the stronger the effect, and we plan to investigate that at a later date.
“Telomere length is linked to longevity, and if our results hold up in future studies, it may be that regular sexual intimacy can lead to a longer life.