Three days, five days, a week… The number of days one goes without washing their hair and just getting by with dry shampoo has become a point of pride these days. We even hear colleagues brag about how many workouts their most recent blowout has lasted through. (Congrats?) But even though over-washing is indeed harsh on strands and can strip their natural moisture, experts tell us that, depending on your hair texture, there actually can be such as thing as going too long between suds. Without regular shampooing hair can become lackluster, smelly, and worse. So, today, in honor of National Shampoo Day (which falls on either the twentieth, twenty-first, or twenty-second day of October—the true date of the made-up holiday is a point of contention in the beauty industry), we asked hairstylists, colorists, dermatologists, and hair experts for the quick and dirty on coming clean.

1. Shampoo Is, You Know, Basic Hygiene

How do you like the sound of “microfauna” in your hair? Us, neither.

“Sebum, which is nature’s hair conditioner, is constantly secreted out of the scalp and onto the hair, and it will build-up without washing. This can leave the hair looking and feeling oily, but worse still, this sebum can act as a food source for microfauna whose action can lead to dandruff and, in extreme cases, can cause hair loss. In addition, product build-up residue from conditioning and styling ingredients will also occur without washing, which collectively can dull the hair and weigh it down. A good cleaning with a good shampoo will prevent all of these issues. In short, this new ‘no poo’ movement really is a crock of …. well, poo.”— Trefor Evans, cosmetic chemist and technical consultant to the hair-care industry

2. It Acts Like Skin Care

You know what happens when you don’t wash your face—and the same goes for your scalp.

“Dry shampoo will bind and pull the oil away from the scalp, but doesn't remove it and can leave hair dry. That, along with dead skin on the scalp, and hair products, like oils and volumizers, need to be removed intermittently, otherwise they can block follicles and lead to inflammation. Some newer shampoos are even designed to strengthen the hair. My favorite is Julien Farel Hydrate Restore, which includes resveratrol and antioxidants, and sits on the scalp for a few minutes before rinsing.”— Doris Day, a New York City dermatologist