Research suggests men who sleep with women suffer impaired mental performance and increased stress.
The Austrian researchers analysed the sleep patterns of young couples, comparing their sleep and after sleep performance when sleeping alone versus sleeping together with their partner.
Comparing 10 nights in each configuration for each of the couples, they found men suffered increased stress levels and exhibited poorer cognitive test performance when they had not slept alone, an effect observed irrespective of whether the couple had sex together.


Women are also said to have had their sleep and performance impaired by sleeping with their partner, though to a much lesser extent than with men.
As usual with such headline grabbing research, the study is vulnerable to scientific criticism on a variety of grounds, the most crucial being a small sample size and failure to consider confounding variables such as differences in sleeping habits between couples.
A researcher responsible bizarrely suggests communal sleeping is unnatural:

“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.”

Well, personally I am happy to have something to hug when I'm sleeping, how about you? ^^