When men spend the night in the same bed as someone else their sleep patterns are disturbed, even if they do not make love, new research shows.

And this leads to poorer performance in mental agility tests the next day.

However women do not suffer the same problem. They too have disturbed sleep if they share a bed, but tend to get better quality rest when they do finally drop off. This means their brain power remains undiminished.

The finding suggests that if a man has an important day ahead he would be best to head off to the spare room to sleep rather than sharing the marital bed.

The new research, featured in New Scientist magazine (must keep) was presented to the Forum of European Neurosciences in Vienna, Austria, last week.

A team led by Professor Gerhard Kloesch at the University of Vienna recruited eight unmarried, childless couples in their twenties for their study.

He asked them to spend 10 nights sleeping apart and ten together while he monitored their rest patterns at night through wrist monitors and sleep diaries.

The next day he gave them mental ability or 'cognitive' tests.

Even without having sex, everyone had more disturbed sleep when sharing a bed than alone but the effects of this on the brain only affected the men.

According to the journal: "Lack of sleep led to increased stress hormone levels in men and reduced their ability to perform simple cognitive tests the next day."

However women seemed to show no such ill-effects even if their sleep was just as disturbed. The researchers found women were more refreshed than men even when they slept the same number of hours and so concluded they must sleep more deeply.

The study also found sharing a bed can impact on dreams. Typically women remembered more of their dreams after sleeping alone while men recalled the most after sex.

Interestingly, most men were not aware that sharing their bed disrupted their sleep - and in fact believe having a partner there helped them drop off.

Women typically felt they slept better when alone, although this makes no difference to their mental performance the next day.

Professor Jim Horne, who runs the Sleep Research Centre at Loughborough University said: "Men would need a really bad night's sleep for their brain's performance to be impaired the next day.

"Also, couples who share a bed for years get used to each other so much that if they then spend a night apart they actually have a worse night's sleep alone than when together."