A US study says that a good way of improving the quality of a man's sperm and thereby increasing his fertility is for him to sleep naked at night and wear boxers during the day.
This study from the US National Health Institute of Child Health and Human Development and Stanford University in California was presented at the annual meeting of the American Society for Reproductive Medicine, which is being held from 17 to 21 October in the US city of Baltimore.
Its findings showed that underwear that is too tight can affect sperm quality and reduce the likelihood of reproductive success.
To arrive at these conclusions, the team of researchers followed 500 men for 12 months, tracking their choice of underwear during the day and at night and recording their sperm quality data.
They discovered that those who wore boxers during the day and slept in the nude had a 25% lower risk of DNA fragmentation in their sperm, which can affect sperm quality, compared to men who usually wore tight underwear during the day and night.
"Among men in the general population attempting pregnancy, type of underwear worn during the day and to bed is associated with semen quality," said Dr Katherine Sapra, the study's lead author. "Reducing exposure for bed decreases DNA fragmentation and better quality parameters are observed in men wearing boxers during the day and none in bed".
The study suggests that men who want to have the best chance of conceiving should take this into account, although it also mentioned that additional research is required.
Previous studies have shown that heat exposure and tight underwear had a negative impact on fertility by destroying part of the sperm cells.
But this is the first study to link clothing habits, day or night, with sperm quality.
France's National Institute for Health Surveillance (InVS) undertook a vast survey between 1989 and 2006, which was published in the December 5, 2012 edition of the journal Human Reproduction. It showed that sperm count had decreased by 30% in France and the number of spermatozoa was falling by 2% each year. Sperm morphology was also increasingly atypical.
In the UK, 40% of men sleep naked according to a survey published in 2014 for the textile brand Cotton USA.