Science’s advice to men looking to have kids in the coming years: ditch the PJs.

In a new study about male fertility, researchers at the National Institute of Child Health and Development and Stanford University suggest men who sleep naked have an improved sperm count versus those who don’t. Additionally, men should wear boxer shorts instead of tight, confining briefs during the day.

In the study, researchers closely monitored 500 men for their daily underwear choices over the course of 12 months, the Independent reports, measuring their sperm throughout the process.  They found that men who slept in the nude and wore boxer shorts in their waking life showed 25 percent less DNA fragmentation in their sperm.

The findings were presented at the annual meeting of the American Society for Reproductive Medicine in Baltimore this week.

“Among men in the general population attempting pregnancy, type of underwear worn during the day and to bed is associated with semen quality,” said lead author Katherine Sapra of the NICHD. “Reducing exposure for bed decreases DNA fragmentation and better quality parameters are observed in men wearing boxers during the day and none in bed.”

Heat also played a role in previous scholarship measuring men’s infertility, associating a decrease in sperm count with laptop usage.

“We have known for some time that men who increase the temperature of their testicles, either through heat exposure at work or by wearing tight underwear, have poorer semen quality compared to men whose testicles are cooler,” Allan Pacey, a fertility expert and professor of andrology at the University of Sheffield, told the Telegraph.

“What has never adequately been shown is whether men can improve things by changing the choice of their underwear, but this study—although quite small—goes some way to suggest that is true,” Pacey added.

Hanes and Fruit of the Loom have yet to release a statement regarding the study.