Not getting enough sleep can reduce a man’s fertility, researchers said yesterday.

Getting less than six hours of sleep a night reduces a man’s chances of getting a woman pregnant by 43 per cent, they found.

However the Boston University study of 695 couples also found that getting too much sleep was almost as bad for fertility – more than nine hours a night reduces chances of having a baby by 42 per cent.

The study looked at men aged 21 or older and women aged between 21 and 45 who were undergoing IVF treatment. The researchers assessed how much sleep the men got and checked every eight weeks for up to 12 months until the couple became pregnant.

Lead author Dr Lauren Wise said: ‘Those men who sleep seven to nine hours a night have the lowest risk of adverse health outcomes.

‘There has been a growing body of literature of the relationship between sleep quality and testosterone which is critical for male sexual behaviourreproduction.

‘Sleep problems specifically have been associated with lower sperm concentrations, total sperm count and per cent of normal sperm morphology (shape), as well as decreased testosterone levels, but no studies have looked at sleep and male fertility.’

Dr Wise, whose study was presented at the annual congress of the American Society for Reproductive Medicine, said sleep disruption affected the production of testosterone, the key hormone that caused sperm to grow.

She added: ‘The majority of daily testosterone release occurs during sleep so we think there may be a hormonal mechanism going on.’

Dan Williams, of the Society for Male Reproduction and Urology, said the research was ‘interesting and provocative’, but added: ‘Do men with sleep problems have impaired fertility because of their sleep problems, or due to other underlying health issues that can affect both their sleep and their fertility?’

Dr Allan Pacey, professor of andrology at Sheffield University, said the exact mechanism which caused damage to sperm through a lack of sleep or too much sleep was not known. He added: ‘Going forward we need a mechanistic study that tried to examine what was different in people who need lots of sleep versus those who need less sleep.

‘It must be something to do with the testicles or hormonesin the brain that make the testicles work. It’s not too difficult a system to understand.’

Couples using personal lubricants during sex have been warned that many brands may harm sperm.

Researchers found that K-Y Jelly was toxic to sperm in a laboratory after one hour at 10 per cent concentration. Durex lubricant was less toxic – at 24 hours, 84 per cent of sperm were alive.

Two brands, Control and Velastisa, had no toxic effect, the University of the Valencia researchers found.

Owen Davis, president of the American Society for Reproductive Medicine, said: ‘The popularity of personal lubricants is increasing and there are significant numbers of women using them who are actively trying to conceive. We need to make patients aware that their choice may affect their chances of becoming pregnant.

‘While the study clearly shows that some lubricants are extremely toxic to sperm, more research is needed to see whether they affect sperm function before we can say that any of these products are fertility friendly.’