The first evidence that sexsomnia - the bizarre act of having sex while asleep - could be genetic has been found by scientists at Victoria University.
In his 16 years as a sleep expert, Gerard Kennedy, an associate professor in psychology, has met a man, who an hour after falling asleep would roll over and begin "rough, mechanical" sex with his wife. The next morning he would have no recollection of the act.
A few suburbs away, his adult son was doing the same, much to the chagrin of his girlfriend.
"This could be the first case in the world where we can see it runs in families," he said.
In both men, the sex started about 50 to 70 minutes after falling asleep, always when they were in the first deep stage of sleep, before dreaming began.
Both men were "rough and mechanical with no concerns for their partner's position or comfort" and neither could be deterred from their mission, even if they were yelled at or hit, said Kennedy.
The part of the brain that controls movement is gradually switched off during sleep as a child becomes a teenager, which is why many children "grow out" of sleepwalking or night terrors.
But in some teenagers that never happens, leaving them as parasomniacs with inappropriate arousals during sleep, causing them to walk, eat or have sex.
After 20 years of sexsomnia, the older man's wife left him. He sought help because he was worried that a new partner might find his behaviour equally disturbing.
Both men have been prescribed a short-acting benzodiazepine, which sedates them enough in the early stages of sleep to stop the behaviour.
A patient, while on a trip with his mates, claimed he was asleep when he visited a nightclub, drank alcohol and took a woman back to his hotel room for sex.
"His girlfriend believed him, but I didn't," the Sydney Morning Herald quoted him as saying.
This unusual case would be presented at the Australasian Sleep Conference in Christchurch.