The researchers interviewed eight unmarried couples in their 20s who were asked to spend 10 nights with their partners, and 10 nights alone. After each night's sleep, scientists checked the stress hormone levels of both the men and the women, and then asked them to take simple cognitive tests.
While most men reported a better night's sleep with their partner, their test results were better on nights they slept alone. According to Professor Gerhard Kloesch, both men and women suffered disturbed sleep when they went to bed with partners, but when they eventually get to sleep, women slept more deeply with their partners.
"It's not surprising that people are disturbed by sleeping together," said Dr. Neil Stanley, a sleep expert at the University of Surrey. "Historically, we have never been meant to sleep in the same bed as each other. It is a bizarre thing to do."
The morning after sleeping with a partner, women's stress hormone levels did not suffer as much as men's, but they still reported better sleep and recalled more when they slept alone. Male recall levels were best on mornings after making love to their partner.
Stanley suggested that women adapt better to a partner because they have many life challenges that disturb sleep, whether they are with a partner or not, including child rearing, menstruation and menopause.
"Sleep is the most selfish thing you can do and it's vital for good physical and mental health," Stanley said. "If you are happy sleeping together that's great, but if not there is no shame in separate beds."
However, Stanley also says that separation could have an adverse effect in couples. "If they have shared their bed with their partner for a long time they miss them and that will disturb sleep," he said.