I’ve always lived my life by the Two Night Pajama Rule—the unofficial (and unscientific) belief that sporting the same PJs for consecutive sleeps is fine. Just fine. But now that curiosity has led me to consult the experts, I’ve discovered that every two-night stand I’ve had with my nightgowns, boxers, and T-shirts has been a filthy lie. Or, to state it in a less dramatic manner, in some situations it’s as gross as wearing the same socks for days on end.

Leanne Stapf, COO of The Cleaning Authority, paints me a powerful visual of why my worn-to-threads “Strand Bookstore” T-shirt should ideally land in the hamper after one overnight wear. “Many people assume that pajamas don’t get dirty because they aren’t worn outside and only when sleeping. However, that’s not the case,” she says. “In addition to the buildup of overnight perspiration, our skin is constantly shedding. Those dead skin cells coupled with the germs accumulated from daily activities are now being transferred to your pajamas.” That’s extra true if you have a full, active day and don’t shower before slipping into your jammies. In this case, the evening following will call for a new set, no questions asked. 

When you’re considering whether or not any garment (nightwear or otherwise) needs to be washed, Clean My Space blogger Melissa Maker has a hot tip: the closer something lies to your skin, the more often it needs a spin in the washing machine. This proves especially true of clothes covering your nether regions (because, duh). “If you sleep without underwear, or just wear underwear to bed, then the bottoms should be changed after one use,” says Maker. But if you’re wearing pants or shorts over your skivvies, it’s not the end of the world if you go a few days without swapping out your PJ bottoms—the undies will provide a buffer.

Specific lifestyle and hygiene choices should dictate how often you swap your PJs out, too. If you tend to wake up soaked in sweat (raises hand), then you’ll definitely want to wear a fresh set each night. “It’s a good reminder that we should be sleeping in a cooler climate and with the right linens and bedding for the season to control how much we sweat and how comfortable we are at night,” notes Maker. If you wake up bedecked in particularly gnarly clothes, optimizing your bedroom environment might help keep your laundry pile from reaching hazardous heights. (And you’ll probably sleep more soundly as a bonus.)

TL;DR: The Two Night Pajama Rule is at least somewhat legit—as long as second-day underwear and perspiration-soaked flannels aren’t part of the equation.

More sleep intel? Here’s how to use somnolent foods to your advantage, and the real difference between sleep and rest.