Couples are happiest when they have sex once a week, a study has found.

The findings challenge the popular notion that when it comes to sex, the more you have the better.

Scientists analysed data collected at three time points over 14 years from more than 2,400 married couples in the US.

Couples reported increasing satisfaction with their relationships as sexual frequency increased up to once a week. Having sex more often than that showed no noticeable benefit.

Lead researcher Dr Amy Muise, from the University of Toronto-Mississauga in Canada, said: "Although more frequent sex is associated with greater happiness, this link was no longer significant at a frequency of more than once a week.

"Our findings suggest that it's important to maintain an intimate connection with your partner, but you don't need to have sex every day as long as you're maintaining that connection."

The study did not establish whether sex made couples happier, or whether being in a happy relationship led to more sex.

No association was found between sexual frequency and well-being in single people, said Dr Muise. The findings were specific to people in romantic relationships.

Once-weekly sex tended to be the typical frequency rate for established couples, according to the research published in the journal Social Psychological and Personality Science.

Dr Muise added: "Our findings were consistent for men and women, younger and older people, and couples who had been married for a few years or decades."

Another study by the same researchers suggested that sex is more strongly associated with happiness than money.

An online survey was carried out among 138 men and 197 women in long-term relationships. There was a bigger happiness difference between those having sex once a week and less than once a month than between people with higher and lower incomes.