From the BBC:
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.
More here. [Thanks to Kris Kotarski.]