How much sex is enough? Just once a week is all it takes for optimal happiness among married heterosexual couples and those in long-term relationships, said a US study Thursday.

The findings are based on surveys of more than 30,000 Americans collected over four decades, and are published in the journal Social Psychological and Personality Science.

"Although more frequent sex is associated with greater happiness, this link was no longer significant at a frequency of more than once a week," said lead researcher Amy Muise, a social psychologist and postdoctoral fellow at the University of Toronto-Mississauga.

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

Researchers noted that the study was not designed to show cause-and-effect, so it remains unknown whether happiness leads to weekly sex, or if weekly sex arouses more joy in life.

The study was also limited to people in romantic relationships, not single people.

"In fact, there was no association between sexual frequency and well-being for single people," said Muise.

The findings were also consistent across age groups, gender and the length of the relationship -- whether months or decades.

Muise said couples should discuss whether their sexual needs are being met, rather than simply press for more sex.

"It's important to maintain an intimate connection with your partner without putting too much pressure on engaging in sex as frequently as possible," she said. afp