Wearing boxer shorts leads to better quality sperm than wearing briefs, fertility researchers have found.
Sleeping naked or wearing pyjamas at night also appears to lead to better-formed sperm than keeping underwear on in bed.
Scientists have long known that men who keep their testicles cooler than the rest of their bodies — the optimal difference is thought to be about one or two degrees celsius — tend to be more fertile.
Researchers from the American National Institutes of Health recruited 501 couples who had decided to try to have children. Those who wore boxers during the day and nothing at night scored lowest for DNA fragmentation, suggesting that keeping their testicles cool had preserved as much of the genetic information in their sperm as possible. DNA can lose its structure when it is exposed to heat.
About 40 per cent of fertility problems are estimated to be due to male infertility. The first results, which will be presented tomorrow, appear to confirm NHS advice that men should avoid wearing tight underwear if they are trying for a baby.