You brush. You floss. You swish some burning mint-laced liquid around in your mouth until it hurts. You go to bed with an oral hygiene gold star, and you wake up with white gloop connecting your lips and some vile odour emanating from it. WTF happens in our mouths while we sleep?
The short answer is that bacteria love the wet, warm, Petri dish you've provided them. The multiplying bacteria release stinky compounds (some 600 or so of them) that make a mess of the place, and there's not a whole lot you can do about it.
Good dental hygiene is about the best head start you can get. Toothpaste and mouthwash create an unfriendly environment for bacteria attempting to multiply with ingredients like xylitol, triclosan, and essential oils. But the effects of these outside ingredients don't last. With a little time, your mouth flushes away the ingredients that keep bacteria at bay, setting the stage for rapid microbial growth.
So far, the mouth situation is pretty much what we'd expect after we brush our teeth. But why is it so much worse at night? We drink coffee and eat garlic and yet it's the morning that produces the most startling breath stench. That's because there's an important element that plays a part in creating bad breath while we're sleeping: saliva — or, rather, the lack of it. Saliva production slows significantly when we're snoozing, so the liquid that's typically responsible for diluting the bacteria or ferrying it down the long slide to our gut is on break. The result is bacteria left alone to multiply for hours, which results in an outsized concentration of components that are cooking up something nasty.
We're not just talking about a few stinky compounds. Scientists estimate that there are more bacteria hanging out in one mouth than there are people on earth. And different types colonise different areas. So a really good tongue brushing isn't going to do much for the stuff that hangs out underneath-or on the roof of your mouth either.
And smell it does. Inside your mouth, microbes are constantly perishing and self-replicating. According to the book Breath: Causes, Diagnosis, and Treatment of Oral Malodor, "The [microbe] decomposition results in the production of a foul-smelling odour. This off gassing in the mouth is a similar process to that occurring at a waste disposal site." No wonder people don't want to take a whiff.
See, bacteria gain energy via amino acid and protein digestion. Some of the amino acids digested contain sulphur, which is released when the bacteria process it. (Sulphur is one cause of unpleasant smells coming out of both ends, so it seems.) But it's not just one smell that makes breath bad; it's a bunch of yucky gasses working together in concert. Again, from the Breath book: "cadaverine (corpse odour), hydrogen sulphide (rotten egg odour), isovaleric acid (sweaty feet odour), methylmercaptan (faecal odour), putrescine (decaying meat odour) and trimethylamine (decomposing fish odour)… are components of human bad breath." The tongue houses nearly all the worst offenders.
In other words, your mouth spends all night whipping up a putrid bouquet for the morning. If your partner isn't into your unique sweaty-feet-plus-corpse-odour offering, the only good news is that it only takes humans about five minutes to adapt to the smell. But would we really want to?
Rachel Swaby is a freelance writer living in San Francisco.
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