The condition of sexsomnia, the bizarre act of having sex while asleep, could be genetic, according to evidence found by an Australian researcher.
Gerard Kennedy, an associate professor in psychology at Victoria University, who studied four sexsomnics in 16 years, has found an unusual case of a father-son duo.
"This could be the first case in the world where we can see it runs in families," he was quoted as saying by The Sydney Morning Herald. Kennedy will present the case at the Australasian Sleep Conference in Christchurch Saturday.
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 begins.
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, Kennedy said.
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, he said.