The phrase “whitens your teeth while you sleep” has become shorthand in some circles for the overblown promises made by advertisers. But what’s the source of this phrase? I assume that there was some product that used it as an advertising slogan, but what was it?
I was going to link to the Ask MetaFilter answers but I think you’ve probably read them.
Actually, I was the one who posted the question over there, too. Hopefully the SDMB can help find a more conclusive answer. 
Well, there are a few products out there which do whiten teeth while one sleeps, but if you’re just asking about the age-old meme, then you’re barking up the wrong tree. There are two memes being smooshed together here: (1) “Whitens teeth and freshens breath” are the claims of many a toothpaste manufacturer, because both are things that the FDA will let them say; (2) “while you sleep” is a common claim for many products that need to be left on overnight, including the earliest example I can remember off the top of my head, which is oven cleaner – remember “I’m cleaning my oven…while I sleep”? No? (damn, i’m getting old…)
‘I knew a man who actually saw the Jabberwock once. Turned his teeth snow white overnight. shudder’
There were TV ads for EZ-Off oven cleaner back in the 70’s with a woman getting ready for bed saying “I’m cleaning my oven - while I sleep!” I guess you let the stuff soak in over night and then just wiped the gunk out in the morning. That’s about the earliest “while you sleep” reference I can think of, though it has nothing to do with teeth.
A mutual friend of Slithy Tove’s?