Snoring, nightmares and "sexsomnia" – having sex while asleep – are among topics covered by sleep experts in Christchurch this week.

More than 500 sleep researchers, clinicians and scientists from around the world are beginning a three-day conference today covering such issues as why one in four New Zealanders do not get enough sleep, the trouble with nightmares, and why severe snoring may damage the brain.

The Australasian Sleep Association conference will also hear a presentation on "sexsomnia" where people – nearly always men – engage in sex while asleep and unaware of what they are doing.

Sydney sleep medicine physician Dr Peter Buchanan said the condition was not common.

"You hear about it occasionally. Usually because there's concern from the partner this is happening and all being done on automatic pilot without normal sensitivity.

"People may feel abused or may just tolerate it because it's their regular partner."

Buchanan said there had been reports where the woman reported the sex being different than when the couple were awake.

The condition has been argued as a defence in several sex cases overseas.

Researchers from Melbourne's Victoria University will present case studies of a father and son who both suffered from sexsomnia. Both men were treated successfully with drugs.

Australasian Sleep Association president Craig Hukins said adequate sleep was critical for good physical and mental health but was often neglected, leaving people fatigued and at risk of injuries and health problems.

'SOMETIMES HE'D WAKE UP HALFWAY THROUGH'

A New Zealand woman whose partner suffers from sexsomnia said initially she did not believe him.

"At first I thought he was using it as an excuse, I mean who really wants to be woken up in the middle of the night?

"When it first happened, in the morning he'd say he couldn't remember. I thought he's just using it as an excuse."

However, the woman, who did not want to be named, said she soon realised he was genuine. "He was doing other things in his sleep so I realised he was telling the truth."

He was a restless sleeper who frequently talked in his sleep and who sometimes got out of bed and walked around the house. The woman said the sleep sex happened about once a week. "He'd wake me up by cuddling me. Sometimes he'd wake up halfway through and be really surprised.

"Like, `how did this happen?"'

She did not realise sexsomnia was a condition.