How often you should wash your bed sheets, according to a microbiologist — and what happens when you don't

woman sleeping

We spend more than a third of our lives in bed — but that place can quickly blossom into a "botanical park" of bacteria and fungus, according to New York University microbiologist Philip Tierno.

If left for too long, the microscopic life within the folds of our bed sheets can even make us sick, Tierno told Business Insider.

To stem the invisible tide, he said, sheets should be washed once a week.

The recommendation is also emphasized by the authors of a study published November 30 in the Journal of Allergy and Clinical Immunology. After testing thousands of American homes, the researchers found that more than 90% of them had at least three detectable allergens.

When these allergens lurk in places where your mouth and nose nestle right up to them, they can trigger sniffing and sneezing regardless of whether you have a known allergy or not.

"Even if you don't have allergies per se, you can have an allergic response," Tierno said.

Another reason so many of these microbes can lurk in our beds is that we keep them warm and moist simply by sleeping in them. Humans naturally produce some 26 gallons of sweat between our sheets every year. When it's hot and humid, this moisture becomes what scientists call an "ideal fungal culture medium;" our pillows alone can house as many as 16 species of fungus each.

In addition to the fungi and bacteria that come from human sources (including sweat, sputum, and anal excretions), beds also teem with foreign microbes like animal dander, pollen, soil, lint, dust mite debris and feces. They can also contain remnants of the finishing agents used to produce sheets.

All that gunk can become what Tierno calls "significant" in as little as a week. So he recommends doing yourself a favor and washing your sheets.

"If you touched dog poo in the street, you'd want to wash your hands," Tierno said. "Consider that analogous to your bedding. If you saw what was there — but of course you don't see it — after a while you have to say to yourself, 'Do I want to sleep in that?'"

Julia Calderone wrote an earlier version of this post.

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