Sharing a bed with someone could temporarily reduce your brain power - at least if you are a man - Austrian scientists suggest.
Bed sharing disturbed sleep quality
When men spend the night with a bed mate their sleep is disturbed, whether they make love or not, and this impairs their mental ability the next day.
The lack of sleep also increases a man's stress hormone levels.
According to the New Scientist study, women who share a bed fare better because they sleep more deeply.
Professor Gerhard Kloesch and colleagues at the University of Vienna studied eight unmarried, childless couples in their 20s.
Each couple was asked to spend 10 nights sleeping together and 10 apart while the scientists assessed their rest patterns with questionnaires and wrist activity monitors.
The next day the couples were asked to perform simple cognitive tests and had their stress hormone levels checked.
Although the men reported they had slept better with a partner, they fared worse in the tests, with their results suggesting they actually had more disturbed sleep.
Both sexes had a more disturbed night's sleep when they shared their bed, Professor Kloesch told a meeting of the Forum of European Neuroscience.
But women apparently managed to sleep more deeply when they did eventually drop off, since they claimed to be more refreshed than their sleep time suggested.
Their stress hormone levels and mental scores did not suffer to the same extent as the men.
But the women still reported that they had the best sleep when they were alone in bed.
Bed sharing also affected dream recall. Women remembered more after sleeping alone and men recalled best after sex.
Dr Neil Stanley, a sleep expert at the University of Surrey, said: "It's not surprising that people are disturbed by sleeping together.
"Historically, we have never been meant to sleep in the same bed as each other. It is a bizarre thing to do.
"Sleep is the most selfish thing you can do and it's vital for good physical and mental health.
"Sharing the bed space with someone who is making noises and who you have to fight with for the duvet is not sensible.
"If you are happy sleeping together that's great, but if not there is no shame in separate beds."
He said there was a suggestion that women are pre-programmed to cope better with broken sleep.
"A lot of life events that women have disturb sleep - bringing up children, the menopause and even the menstrual cycle," he explained.
But Dr Stanley added people did get used to sharing a bed.
"If they have shared their bed with their partner for a long time they miss them and that will disturb sleep."