Crawling into bed after a long day is a moment of sweet relief, especially if your sheets are clean and crisp. After a few weeks of sleeping in them, however, you’ll probably start to notice that vague musty smell emanating from your pillowcase, and your bed sheets somehow won’t be as soft as they were before. What you don’t see are the millions of microscopic inhabitants crawling in your cotton bedding, sleeping right there beside you.

According to a YouGov poll, about 33 percent of British people wash their sheets every week, while 35 percent wash them every two weeks. Eight percent launder their linen every three weeks, and 10 percent do it every four weeks. It’s even worse for bachelors: A 2013 study found that single men only washed their sheets four times a year, due to not feeling any “need” to bother with it.

But washing your sheets so infrequently may be harmful to your health. Crumbs or stains from eating late-night snacks aren’t the only invaders in your bed: Your sheets are likely filled with fungi, bacteria, dust, dirt, pollen, and other allergens that settle in linen and mattresses over time. In humid summer months, your sweat seeps into your bed sheets during the night, along with other bodily fluids. Dead skin, cosmetics and oils, and dirt on your feet rub into your sheets and settle deep inside your mattress where they can cause allergies and itchy throats, and in extreme cases, infection. There’s also always a risk of bed bugs in uncleaned sheets, which tend to be attracted to red sheets more than other colors (possibly due to the fact that bed bugs themselves are red, and they tend to gravitate towards groups).

The conclusion? Wash your sheets at least once a week, or once every two weeks, as most of the above allergens and microbes accumulate in that time frame. Whereas in the past you might have been lazy enough to leave sheets on your bed for a month or more, remember the good feeling of showering and slipping under fresh, clean sheets to motivate you. You may sleep more soundly if you do.