Beauty sleep? More like baby-making sleep. Women may feel more tired or sleep-deprived than men, but a new Danish study found that lack of sleep can take a major toll on guys, too — by reducing their fertility. Read on to learn why dudes should hit the hay earlier in order to keep their little swimmers healthy.

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What’s the Deal?

To determine how sleep affects sperm health and concentration, scientists at the University of Southern Denmark surveyed 953 healthy young Danish men between January 2008 and June 2011Association of sleep disturbances with reduced semen quality: a cross-sectional study among 953 healthy young Danish men. Jensen TK, Andersson AM, Skakkebaek NE, Joensen UN, Jensen MB, Lassen TH, Nordkap L, Olesen IA, Hansen AM, Rod NH, Jorgensen N. Department of Environmental Medicine, Faculty of Health Sciences, University of Southern Denmark, Winsloewparken 17, Odense, Denmark. American Journal of Epidemiology. 2013 April 7.. Each man was studied just one time. The subjects provided blood and semen samples, underwent a physical examination, and filled out a questionnaire describing the previous month’s sleep habits, and then the results were in: The men who were major night owls or who had trouble falling and staying asleep had sperm counts that were an average of 29 percent lower than those of the dudes who fell asleep as soon as their heads touched the pillow.

The researchers also found that on average, men with unhealthy sleep habits had smaller testicles than the men who regularly got seven to nine hours of rest each night. The scientists aren’t sure if this form-fits-function parallel was a direct result of sleep deprivation or whether it’s related to baby-making abilities in other ways. Based on the overall youth and health of all the men involved in the study, the researchers were able to conclude that quality and duration of sleep has a big effect on sperm count, which determines male fertility.

Why It Matters

Although this is the first study to directly observe how sleep habits affect fertility, it’s not the only time researchers have looked at the connection between our two favorite bed-centric activities (we’re talkin’ sleep and sexytimes). A few years ago, University of Chicago researchers found that restricting sleep is a surefire way to squash testosteroneEffect of 1 wek of sleep restriction on testosterone levels in young healthy men. Leproult R, Van Cauter E. Department of Medicine, University of Chicago, Chicago, IL, USA. The Journal of the American Medical Association. 2011 June 1; 305(21):2173-4.. After just one week of sleeping five hours a night or less, the young men involved in Chicago study had significantly lower testosterone levels (up to 15 percent, in fact) than their well-rested peers.

But why is having plenty of testosterone so important? Haven’t we evolved past the He-Man view of masculinity? In actuality, it’s not all about the muscles — the sex hormone affects body processes from red blood cell production, to fat distribution, to concentration, to sexual performance and, yup, sperm production.

Is it Legit?

Most likely. Previous research has suggested that if there were such a thing as a “magic bullet” for health, getting plenty of snooze time might be it. Past studies have shown that getting plenty of sleep can help people lose weight, reduce workplace stress, and even drink less alcohol. Since frequent late nights (or nights full of tossing and turning) put the brakes on testosterone production, it makes sense that staying up late also reduces overall quantity and quality of sperm.

What’s an insomnia-afflicted but fertility-minded dude to do? The best way to ensure a plethora of sperm is to begin developing healthy sleep habitsto prevent and deal with bouts of insomnia or other sleep issues.

Regardless of gender, would fertility concerns motivate you to hit the hay earlier? Why or why not? Tell us your thoughts in the comments below or tweet the author @SophBreene.