Imagine finding unexplained condoms around your house and then waking up one night to find your partner having sex with a stranger.
It might sound like an affair, but what if your "cheating" partner was fast asleep during the act?
The phenomenon, called sleep sex, was described to doctors at a meeting in Australia.
Sleep physician Peter Buchanan, from Sydney's Royal Alfred Hospital, described this real life case.
Mr Buchanan told the Australasian Sleep Association how a patient of his, who was a respectable middle-aged woman with a steady partner, would leave the house while sleepwalking and have sex with strangers.
The woman was totally unaware of her double life until her partner became suspicious and found her engaged in the act.
"He was aware of some sleepwalking and there was circumstantial evidence, including the unexplained presence of condoms around the house," Mr Buchanan told the conference.
Mr Buchanan ran a series of tests on the woman and diagnosed her problem - sleep sex.
This is a condition completely distinct from sleepwalking and is a form of sleep disorder called REM behavioural disorder.
Normally, when a person enters a phase of sleep called the REM (rapid eye movement) phase, in which we can often dream, our bodies are immobilised.
In the case of sleep sex, this doesn't happen and the person can act out their dreams.
UK sleep expert Neil Stanley from the Human Psychopharmacology Research Unit at Surrey University explained.
"In some people, it can be genetic or in others it is triggered by alcohol or stress, they do not lose the muscle tone.
"So that means whatever they are dreaming about, which at that time is their reality, they actually do."
He said if what the person was acting out fitted with the dream they might not wake up.
"If you are lying there dreaming about having sex with your wife and you just happen to be having sex with your wife then there's nothing there to stop the dream. You don't perceive that as wrong."
He said there were documented cases where people had committed murders in their sleep.
"There is a case in English law where a guy was beating his girlfriend over the head with a video recorder and she was screaming at him."
He said the man only woke up when the girlfriend said "I love you".
"Probably that was such a stupid thing in his dream that did not fit that it woke him up.
"It's like when you hear the phone in the middle of your dream and you wake up."
He said people would not necessarily remember what they had done while they were dreaming.
"You only remember your dreams if you wake up during them.
"But even then, it can be very difficult to comprehend that you have done what you dreamed about."
He said finding out what was the trigger of the REM behavioural disorder and eliminating it could help.
Other treatments include medications to induce sleep or reduce muscle tone.
Mr Buchanan said almost half of all sleep sex cases were associated with psychological problems and, in the case of the woman he described, psychotherapy helped.
THE STAGES OF SLEEP
Stage 1 - passed through briefly when falling to sleep
Stage 2 - most of the sleeping time is spent in this stage
Stages 3 and 4 - deep sleep, when people can sleepwalk
Stage 5 - REM sleep, when people can sex sleep