Here’s one more reason to get your butt off the couch: Watching too much TV can lower your sperm count, recent research from Denmark suggests.

In the study, men who logged 5 or more hours of TV per day had 29 percent lower sperm concentration and 34 percent lower sperm counts than men who didn’t watch any TV.

Related: 7 Signs Your Sperm Is Healthy and Strong

That dip may make it more difficult to father a child, says study coauthor Lærke Priskorn, Ph.D.(c) of the University of Copenhagen.

Interestingly, men who played on the computer for the same amount of time—another activity where you remain parked on your butt—didn’t experience a drop in those semen parameters.

Researchers aren’t sure yet what it is about TV specifically that’s so harmful to your sperm.

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It’s a more passive activity than using a computer, Priskorn says, and you might take more active breaks from a computer than you would from TV.

It’s possible that all the stillness increases the temperature in your scrotum, which can mess with sperm production, she says.

Related: Why You Should Examine Your Balls Once a Month

The lack of exercise could also be to blame: Antioxidant enzymes sparked by physical activity can help protect your developing sperm cells against damage.

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Or it could just be that guys who watch a ton of TV are also more likely to be overweight or drink excessively, both of which hurt your sperm, Priskorn notes.

Your move: If you’re trying to become a dad, it doesn’t hurt to cut down on couch time. And when you do sit down to catch up on Narcos, take breaks between episodes to do something active—like these 5 Stretches You Should Do While You’re Watching TV.

This article originally appeared on MensHealth.com.