Want to give your little swimmers a fighting chance? Get a good night’s sleep, suggests a new study in the journal Fertility & Sterility.

Researchers analyzed the sleeping habits of almost 700 couples over a full year, and found that men who slept less than six hour per night were 31 percent less likely to impregnate their partners than men who slept between seven and eight hours. An even more dramatic difference was found in men who slept nine hours or more, who were 49 percent less likely to impregnate their partners.

The researchers add that it’s not totally clear yet how sleep length is impacting sperm, as the study used pregnancy to determine male fertility and not data on sperm counts or motility

Researcher Lauren Wise of Boston University surmises it’s possible that too little sleep reduces your release of testosterone, which is essential in sperm production. It’s less clear why long sleep might be hurting fertility, but may have something to do with poorer sleep quality, which can also chisel away at testosterone.

In fact, men who reported having trouble sleeping through the night were 28 percent less likely to get their partners pregnant than those who were sans-insomnia. The researchers add that it’s possible men with poor sleep quality also indulge in other sperm-harming habits, like smoking, drinking or carrying too much extra weight.

Wise recommends those looking to become fathers sometime soon should start adjusting their sleeping habits roughly three months before they want to conceive. This is because sperm mature in roughly 72 days, meaning it could take a little over two months to see any improvement to your fertility.

h/t Men’s Health