Women may need more sleep than men, but couples looking to brew up another baby may want to split the difference. New research out of Harbin Medical University in China suggests that men who go to bed after midnight have lower sperm counts and that the sperm they do have tend to die much sooner than in men with earlier bedtimes.

The study, published in the Medical Science Monitor, involved 981 Chinese men who were randomized to go to bed and wake up at specific times. By the end of the study, the men who slept for less than six hours or more than nine had the lowest sperm counts and sperm survival rates. The authors suspect that late bedtimes and inadequate rest hurt sperm by increasing the levels of antisperm antibodies—proteins that can kill off a would-be dad’s swim team.

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Of course, this could be concerning to baby-makers in their thirties and onward, who are neurologically predisposed to shittier sleep as they age. However, the findings come with the important caveat that people who get little sleep tend to also be at risk for diabetes, obesity, heart disease, hypertension, and mood disorders—health conditions that have also been linked to low sperm count. In other words, it’s difficult to know whether low sperm count is caused by lack of sleep, or whether lack of sleep is just one of many symptoms of a different problem that’s cutting sperm down to size.

“It can’t prove cause and effect,” Lauren Wise, professor of epidemiology and author of a similar study that looked at the sleep patterns of 790 couples, told Web MD in 2016. “It is possible that poor sleep duration could contribute to an unhealthy lifestyle, decreased libido, a decrease in sex, but we tried to control for all those factors.”

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Good sleep hygiene, then, appears to be crucial for one’s overall health, and it follows that sleep is likely an important piece of the fertility puzzle. But it’s not the whole story, and there are frankly an exhausting number of other factors, such as nutrition, to consider. But you can do yourself (and your boys) a favor by starting with a good night’s sleep.