After a long, hard day, there’s nothing like curling up in cotton sheets. But, if you’re like many other people out there, you’re also curling up in dead skin, bacteria, fungus, mites, and fecal matter. Yes, poop.

We polled 1,187 readers on how often they wash their sheets and change out their pillows, and while 44 percent of women said they wash them once a week, 31 percent said they wash theirs twice a month and 16 percent said they do so just once a month. Meanwhile, 32 percent of women said they almost never switch out their pillows.

At minimum, you should wash your sheets once a week, says Philip Tierno, Jr., Ph.D., director of clinical microbiology and immunology at New York University's Langone Medical Center.

That’s because the second you crawl into your sheets, you’re infecting them with your constantly-shedding skin (you lose millions of skin cells per day!), lotions, makeup, sweat, hair, and anything you’ve picked up during your day, like pollen, pet dander, fungal mold, and dirt particles, says Tierno. All come with a not-so-healthy dose of bacteria—for instance, sweat can carry fecal matter and E. coli from your poop shoot to your sheets—which grows the longer you postpone laundry day.

What’s more, the sweat and skin attracts dust mites, and gives them food to munch on and multiply, he says. The thought of cozying up with a bunch of mites is enough to give most women the heebie-jeebies, but the bugs themselves don’t actually hurt you. What can is their poop: It can exacerbate allergies and asthma. And, even if you don’t have allergies, it can cause you to wake up in the morning with red eyes and a stuffy nose, he says.

“A lot of people don’t realize that they spend one-third of their life exposed to these allergens,” says Tierno. That’s a lot of time to be lying in a bed of mites and poop. 

But, even if you wash your sheets once a week, you could still be sleeping on top of a bed of nastiness: Over time, gravity takes over and these particles and bacteria seep their way into your mattress and pillows, he says.

Sure, you could just toss your mattress into the dumpster and buy new pillows, but the easiest way to prevent bacteria from accumulating (and to keep anything that’s already in there from getting to you) is to buy mattress and pillow covers, says Tierno. Look for ones that say they are “impermeable” and that encase the entire mattress, rather than just covering the top of it. Wash them every few months (aim for the change of each season) to get rid of any buildup.